
Pass T J qg-T 
Rnnlr V -; - 
GopghtN" 

COPYRIGHT DEFOSm 



COMPRESSED AIR PRACTICE 



^illllillllllllllllillilllliillllillill^^ Illlillliillii^ 



&■ 



McGraw-Hill Dookfompaiiy 

Puj6Cis/iers qf3oo£§/br 

ElGCtrical World TheEnginGGring and Mining Journal 
LngiriGGring Record Engineering Nows 

Railway A^e G azotte American Machinist 

Signal EngiriQer American Engineer 

Electric Railway Journal Coal Age 

Metallurgical and Chemical Engineering P o we r 

liiiiliillliiiiiliiiillililiiiiM^^^ liliillllilillllilliliiililllllli 



a 




Electric Air Drill in a Granite Quarry. Frontispiece 



COMPRESSED 
AIR PRACTICE 



BY 



FRANK RICHARDS 



McGRAW-HILL BOOK COMPANY, Inc. 
239 WEST 39TH STREET, NEW YORK 

6 BOUVERIE STREET, LONDON. E. C. 

1913 






Copyright, 1913, by the 
McGraw-Hill Book Company, Inc. 



/^-r - ^^>^/</^ 



THE. MAPLE. PKESa. YORK- PA 



©GI,A3 5 78 26 



BEFORE BEGINNING 

The material comprising my little book, "Compressed Air" 
some small portion of which may be recoginzed in the present 
volume, was written twenty years ago. In the two decades 
which have elapsed there has been a great advance in the in- 
dustrial status of compressed air, and a wonderful development 
and extension ot compressed air practice, with many radical 
changes, and the writer in the meantime has had to unlearn 
perhaps as much as he has learned. 

It is only the simple truth that the average power and fuel cost 
of the compressed air now used — compressed by steam power, 
directly or indirectly, as most of it still is — is not more than one- 
third as great, per quantity unit, as it was twenty years ago; 
and perhaps there has been, but concerning this no assertion is 
here made, an equal augmentation of advantage in the useful 
applications of the air. As to the increase in the twenty years 
•in the volume of air more or less compressed and industrially 
employed a thousand fold may be a conservative guess, while 
to say a hundred fold would suggest an excess of caution or a 
deficiency of information. 

It follows that with the vast extension of the field of compressed 
air employment, there has been a corresponding accumulation 
of knowledge concerning it, an increase in the number and variety 
of the things to be told about it, leaving the scope of the inquiry 
as to the possibilities beyond the present more extensive than 
ever. This book just tells what it can, within the permissible 
limits of it, leaving still more than ever unsaid, but perhaps 
suggesting things to be supplied by others who may come after. 

We designedly have to do here with the general rather than 
with the specific, with the approximate rather than the precise. 
The book is intended for the many rather than for the few, for 
those who know little about air rather than for those who mostly 
know all that is known. Those who need more accurate and 
detailed information for practical guidance may perhaps get 
some hints here as to the how, if not so much the where, to look 
for it. 

vii 



viii BEFORE BEGINNING 

There has developed within a very few recent years a special 
line of publications put forth by the builders of air compressors 
and by the manufacturers of the various air actuated tools and 
apparatus. These publications, usually costing the reader 
nothing, well written, excellently illustrated and printed, and 
easy to handle, embody a great variety and reach of definite 
and generally reliable information which it is not the purpose 
of this book to reproduce in any form. Catalog ^'literature" and 
auctioneer talk have their place, but not here. 

Fkank Richards. 
New York, 

November, 1913. 



CONTENTS 

Chapter Page 

Preface vii 

I. Atmospheric generalities 1 

II. Definitions and general information 9 

III. The compressed-air problem 28 

IV. Tables and diagrams for computations in air-compression . 35 
V. The indicator on the air-compressor 51 

VI. Single-stage compression 68 

VII. Two-stage air compression 81 

VIII. Two-stage and three-stage compression 92 

IX. Air compressor regulating devices 101 

X. The drive of the compressor 110 

XI. The turbo compressor 116 

XII. The Taylor compressor — the Humphrey pump 124 

XIII. Power cost of compressed air 140 

XIV. Power from compressed air 151 

XV. The air receiver 161 

XVI. Pipe transmission 171 

XVII. Re-heating compressed air 181 

XVIII. Compressor and receiver fires and explosions 189 

XIX. Side lines for the air-compressor 200 

XX. Gasoline by compression' — liquefied natural gas 216 

XXI. Rock drill developments 223 

XXII. The electric air drill 232 

XXIII. Compressed air for raising water 240 

XXIV. The air lift 252 

XXV. Air for large steam hammers 266 

XXVI. Diving bell and caisson 278 

XXVII. Air jet — sand blast — cement gun 293 

XXVIII. Liquid air — oxygen from the atmosphere 305 

Index 321 



IX 



COMPRESSED AIR PRACTICE 

CHAPTER I 
ATMOSPHERIC GENERALITIES 

As all the air that we come in touch with, or about which we 
know anything, is compressed air, less compressed upon the 
mountain tops, or in the spaces that the aviators tumble from, 
and more compressed in the lowest deeps of caves and mines 
by the weight of the air always above it; if we speak of air at 
all, and without any specification as to its condition, we neces- 
sarily must be speaking of compressed air, and in going through 
the pages which follow it may be well to keep the fact in mind, 
instead of having to remind ourselves of it every time that air 
or the air is mentioned. 

It is not easy to recognize or to appreciate all that the air, 
compressed air if you please, is to the earth as a whole, or to 
comprehend the many and diverse things it is employed to do, 
and which it alone can do, in the active workings of nature. A 
partial running over in our minds of some of its world functions, 
not knowing even yet what they all are, nor any of them with 
any completeness, should crowd our minds with suggestions of 
ways in which we may employ it to advantage, and may lead us 
to regret and wonder that we have not already more fully availed 
ourselves of its ready abiUty to serve us. 

The total weight of the earth's atmosphere is, say, only one- 
millionth of the weight of the earth as a whole; yet that one- 
millionth would seem to be of more value and importance than 
any other specific constituent, however vast, of the entire mass. 
The total volume of the waters of the earth weighs perhaps 300 
times as much as the air, yet the air is still the more important 
and necessary, if, indeed, such a comparison is permissible where 
both are indispensable. 

In the artificial development of power from the active forces of 
nature the windmill as such makes but a pitiful showing when 
compared with the might of the constantly growing host of 

1 



2 COMPRESSED AIR PRACTICE 

water-turned wheels; yet Niagara and Victoria and all the other 
waterfalls, little and big, and the rivers and flowing streamlets 
everywhere, and all the tossing waves which make playthings of 
the ships, are actuated ultimately and entirely by power of which 
the air is the vehicle, and every waterwheel is a windmill in 
disguise. The waters are all lifted from their ocean reservoirs 
by the atmosphere, and are carried by it and delivered to flow all 
over the land and through all the vafleys down to the sea, and 
then the air draws them up and carries and distributes them again; 
and so evermore the air drives all the natural and all the artificial 
activities of the world. The air is the breath of life of the earth, 
as it is of all who dwell upon it. 

Man is constantly and absolutely dependent upon the air for 
his life and for all satisfaction and accomplishment in life. He 
can survive without solid food three weeks, without water three 
days, but without air for only three minutes. Of his catalogued 
senses those of hearing and smell are directly air actuated. The 
eye receives its most vivid and memorable impressions from the 
constantly recurring air-born phenomena of the skies, the chang- 
ing draperies of the clouds all day, the gorgeous climax of the sun- 
set and the sombre draperies of the night; water can act directly 
upon the touch alone. 

Air is the breath not only of man and the land animals but also 
of the plants, the abstracting from the atmosphere of certain 
elements by the one balancing and compensating for that taken 
by the other, both in the process exchanging what they must 
discard for what they most need. By air pressure necessary 
gases are retained in water, in the blood of animals and in the 
sap of plants. 

The air, we learn, is merely a mechanical mixture and not a 
chemical combination of gases, one or the other of its principal 
constituents being abstracted in varying quantities in sustaining 
the functions of universal life, and yet the proportions of the 
mixture are automatically maintained by the balanced operations 
of nature, while to man with a full supply of ingredients and ap- 
paratus it would be impossible to produce a single breath of 
"fresh" air. 

While the air might, in a way, be called a jack-of-all-trades, 
it does not work by makeshifts, but is perfectly equipped for its 
several functions. Perhaps first in the kit which it carries is 
aqueous vapor, and this it keeps constantly active, the meteor- 




5 

*s 

03 
> 

a 

c 

0) 



c3 

a 

OS 

S 
o 
O 

o 
O 



§ 

c3 

d 

O 

O 

IS 
<^ fl 

;» •- 

•5 .S 
1 ^ 

•2 d 

1^ 



ATMOSPHERIC GENERALITIES 3 

ological reports showing the fluctuations of atmospheric humidity 
to have a width of range and a rapidity of change with which 
none of the other reported phenomena can compare; but then the 
normal air carries also carbon dioxide, ammonia, ozone, acid 
compounds of nitrogen and sulphur, and small samples of many 
other gases, with a miscellaneous collection of dust particles and 
of germs and seeds innumerable, all of which are heard from in 
their turn. 

While we all of us must have some respect for the atmosphere, 
and while we may understand in a general way that we couldn't 
get along without it, still many of us have an idea that it does its 
work in a rough, or as we might say, in a careless or unprecise 
way, notwithstanding that it '^gets there" every time; but, as 
man seems to be slowly finding out, some of the littlest work as 
well as the biggest is done by the air. The swift and untiring 
flight of the bird of broad and sweeping wing is impressive, but 
we may not forget that the air is also the free highway for myriads 
of minute organisms down to invisibility and far beyond. 

The functions of the air would seem, from the immediate 
human viewpoint, to be not all nor always beneficent. Not only 
are we dependent upon it for life and health, but it also carries 
disease and, it may be, death, so that now and always the air 
is the most important single matter to which healers and sani- 
tarians can turn their attention. This in actual life would seem 
to be little reaHzed by us, since it is so notorious that we habitu- 
ally are much less fastidious and exacting about the composition 
and condition of the air we breathe than we are about oiu* food 
and drink. Air is always ready to provide the active agencies of 
decay, so that the function of excluding the air from edible 
materials which are to be kept for a length of time is the founda- 
tion of some of our most important industries. 

The air does not always work in the mass, but is employed by 
animated selective agencies in minute isolated portions for deli- 
cate and responsible operations. Let us remember the air 
bladder of the little fish. What more wonderful than the perfect 
poise of a fish in the water? yet the device by which principally 
it is maintained is of the simplest character, but no human 
ingenuity can suggest how the air function could be dispensed 
with. 

The most alert and the swiftest fish of prey with which we are 
familiar, such, for instance, as the pike or the pickerel, seem to 



4 COMPRESSED AIR PRACTICE 

spend much of their time absolutely without change of position, 
watching and waiting in readiness to dash at their prey, as 
upon land the cat watches the bird or the mouse. 

A submerged body charged with life forces, but for the time 
floating inert in the water, is apparently free to move, or rather 
to be moved, in any direction, and that it should not so move is 
probably the most unlikely thing which could be predicted of it, 
yet fish seem to be provided with the means of resisting or of 
counteracting the minutest as well as the strongest of disturbing 
forces, and to be able to maintain a position when so desired 
apparently without effort, and in fact almost automatically, so 
that they often seem to sleep in a running stream as though 
anchored there. 

To retain its position vertically, so that it will neither rise nor 
sink in the water, the body of the fish as a whole must have pre- 
cisely the same weight as the volume of water which it displaces, 
and means must be provided, and they are so provided, for the 
instant adjustment of the specific gravity of the fish body . The 
air bladder of the fish, normally filled with compressed air, is a 
device for the purpose which for simplicity, precision and effec- 
tiveness is perfection. 

The use of compressed air in the bladder of the fish is perhaps 
the most pronounced instance we have of ''compressed air" 
practice entirely unassociated with human agencies. The air 
in the fish bladder must be always under more or less pressure, 
such pressure as would be easily measurable and recordable by 
our gages if only it were accessible. The pressure, indeed, may 
be considerable, as we are only familiar with the bladder after 
removal from the dead fish, and which although still distended by 
sensible air pressure is released from the muscular compression 
which is normal to it in the living fish. The pressure of the air 
in the bladder opposes a constantly maintained muscular tension 
in the body which encloses it. When the muscular tension is 
increased the air is still more compressed, giving the bladder a 
diminished volume, and the entire body of the fish in which the 
bladder is enclosed is correspondingly reduced in bulk, so that, 
while its actual weight remains constant, its weight relatively to 
the weight of the water displaced is increased, and the tendency 
of the fish body is then to sink in the water. 

On the other hand, if the muscles which hold the air bladder 
and its contents in compression are relaxed, the bladder, and then 



1 



ATMOSPHERIC GENERALITIES 5 

the body of the fish, will be distended and the fish will have a 
tendency to rise in the water. When fish are not well and have 
not the muscular strength to maintain the requisite compression 
they float to the surface. If a mermaid belle in a ballroom of the 
deep should faint away she would not fall to the floor, but would 
float to the ceiling and become entangled among the chandeliers. 

The air bladder of the fish has a still more delicate and respon- 
sible function than that of merely maintaining the precise specific 
gravity of the body. The stability of the fish laterally in the 
water is assured by the fact that the air bladder is normally 
located high in the body and the preponderance of weight is 
below it. The longitudinal, or fore-and-aft, trim of the fish is 
more difficult to adjust and maintain, as builders and operators 
of submarine craft have learned. This also the air bladder per- 
fectly provides for, it being of considerable length relatively to 
its diameter and also partitioned transversely or divided into two 
chambers, the muscular control of the fish transferring air from 
one chamber to the other as may be required. The fish floating 
in liquid of uniform weight and density is thus perfectly equipped, 
with sKght local adjustments by the aid of its fins, for maintain- 
ing its position and balance with ease and accuracy. 

The atmosphere is one of the instruments by whose employ- 
ment nature does some of its finest work, and, relatively to earth 
as a whole, its functions are adjusted with micrometric precision. 
A favorite plaything of the writer — in his mind — is an 8-in. 
terrestrial globe. The precise size of it is important here as it 
will enable us to do our thinking to scale, a very necessary par- 
ticular, here and elsewhere, if our thinking is to be of any account. 
Earth is understood to be about 8,000 miles in diameter, and on 
our 8-in. globe, therefore, an inch will represent a thousand miles 
and a thousandth of an inch will represent a mile. This at once 
makes things on earth's surface look very small, and the heights 
and the depths, which make our mountain and valley scenery for 
globe trotters to patronize, become too minute even for detection. 

If there were a fellow big enough to hold earth in his hand like 
a baseball, its surface would, on the same scale, appear to him 
smoother and more highly polished than glass. If we assume our 
8-in. globe to be as smoothly finished as possible all over its sur- 
face, and if we paste upon it a bit of paper a thousandth of an 
inch thick, the thinnest tissue paper we can get, that will of course 
be a mile high, and it will represent the range of altitude within 



6 COMPRESSED AIR PRACTICE 

which 95 per cent, and more of the human race live and move and 
have their being. Dwellers upon the plains and the prairies, 
who know nothing in their actual lives higher than a two story 
or a three story house, may be assumed to keep all their goings 
up and goings down within a vertical range of 50 ft., which is 
about one one-hundredth of the thickness of our patch of tissue 
paper, one one-hundred-thousandth of an inch (0.00001). Den- 
izens of the skyscraper city have a vaster range of altitude. You 
may have an office and do your writing 150 ft. above the side- 
walk, and you may even lunch in a restaurant 300 ft. above the 
curb, and then in your daily life you may boast a vertical range 
of at least 0.00006 in. on our 8-in. globe. 

In this connection we may get the atmosphere, ''free air," 
to do some actual micrometric work for us. The writer has a 
pocket aneroid barometer, which, as everybody knows, is simply 
an air pressure gage, that is another of his personal playthings, 
sometimes used for observations in the elevators of tall buildings. 
This handy instrument will show a measurable difference in the 
pressure of the air with a change of elevation of 25 ft., correspond- 
ing to 0.005 of the thickness of our sheet of tissue paper, or to 
0.000005 in. on the diameter of our 8-in. globe. There are fine 
instruments used by engineers which will indicate a difference of 
altitude of 5 ft., which would be 0.001 of the thickness of our bit 
of tissue paper, or 0.000000125 of the diameter of the earth. The 
difference of air pressm-e for a difference of altitude of 5 ft. would 
be about 0.0025 lb. per square inch. 

We are far enough from having discovered and appropriated 
all the possibilities of air employment for the service of man, and 
where we do employ it our methods are crude and inefficient as 
compared with those observed in the large atmospheric phe- 
nomena. In the easy way in which nature works the air is made 
to do its gigantic tasks by means of comparatively slight changes 
of weight or pressure, of temperature or of humidity. All the 
changes of the weather may be called compressed air effects, 
and yet the normal range of atmospheric air pressures as registered 
by the barometer is not more than half a pound to the square inch, 
with 1 lb. as the limit of variation, while if man undertakes any 
compressed-air work such pressures are worthless. A hundred 
pounds is the common working pressure he employs, while for 
special purposes it must be thousands of pounds. 

Nevertheless it is for us to go on doing the best we can with the 



ATMOSPHERIC GENERALITIES 7 

air we use, hoping to do better and better, and with httle thought 
as to the disturbances we may cause. We are not hkely to 
joggle the universe much. 

Collective humanity, like the individual man, creates for 
itself — makes out of nothing — many unwarranted worries. Our 
supplies of coal and iron are going to give out, and what will we 
do then? Our earth-drawn nitrates for fertihzers are apparently 
much nearer exhaustion than coal and iron, and now that we turn 
directly to the atmosphere to abstract a supply a new worry is 
invented. What will we do when the air fails us? 

It is proper to remember that we are not as big in comparison 
with the earth and its appurtenances as we are apt to think we 
are, and we have little power to disturb its estabhshed arrange- 
ments. We may say that — in very round numbers — the total 
weight of the atmosphere is 5,000,000,000,000,000 tons. That 
would be — in the same style of numerical rotundity — 3,850,000,- 
000,000,000 tons of nitrogen and 1,150,000,000,000,000 tons of 
oxygen. Now, suppose that we abstracted a million tons, or 
a hundred milHon tons, of either, how much would they be likely 
to be missed? 

As a matter of fact we are mechanically abstracting from tho 
atmosphere very small quantities, comparatively, of both nitro- 
gen and oxygen, and these abstractings may so balance each 
other that the relative proportions of the two in the atmosphere 
as a whole may be little disturbed, but even this need not imply 
that we are reducing the total mass of the air by the amount of 
its constituents which we take from it by our modern mechanical 
manipulations. Whatever we seem to get we cannot ''get away 
with" an atom of it We can at the most only borrow, and our 
borrowings are ''returned to stock '^ in spite of us. 

We have to remember nature's automatic and constant restora- 
tive processes. The wonderful increase in our industries in the 
last hundred years has entailed the burning of coal, and now oil, 
in vast and 'ricreasing quantities, and this results in throwing off 
into the atmosphere incalculable volumes of carbon dioxide, 
and this alone might ultimately render life on the earth impossible, 
only that the process does not end with our doings. As we in- 
crease the burden of carbon dioxide in the atmosphere by all 
this active combustion, in addition to the slow combustion in 
our lungs and in those of all breathing animals, we stimulate plant 
life by which the carbon is digested out and the oxygen is returned 



8 COMPRESSED AIR PRACTICE 

to the air to act as a carrier or go-between over and over again. 

It may be safely assumed that the more carbon dioxide there 
is in the air, the more rapid and luxuriant will plant growth 
become, especially the rapid growing annual plants upon which 
we depend for food, these in the aggregate presenting vastly 
greater air contact surfaces than do the more impressive tree 
structures. So far as we can see, the result of artificially increas- 
ing the production of carbon dioxide, thus stimulating plant 
growth, is ultimately beneficial to the human race rather than 
otherwise. 

If the matter is sufficiently looked into it will be found that 
nature's processes provide also for the restoring of nitrogen to 
the air, if by any other agencies it may be abstracted, so that the 
balance of atmospheric constituents is maintained from this side 
also. It is a curious thing that nitrogen, the larger component 
of the air has been less minutely investigated, and the ramifica- 
tions of its functions are less definitely known than those of 
oxygen. 

It is well understood that plant growth persistently abstracts 
nitrates from the soil, and that in successful agriculture these 
must be artificially replaced, but it is coming to be recognized 
that there are certain plants which add to the nitrate constituents 
of the soil, so that these may be made to assist each other; and 
the further we go in our investigations the more do we discover 
these automatic compensations. 

So far as we may think we decipher at all the ultimate plan 
of things we may well believe that the atmosphere is made for 
free and universal use, that any human being may do with any 
portion of it whatever he will, that the entire human race may 
do the sam.e, getting whatever benefit and satisfaction they can 
out of it without the slightest occasion for self reproach or anxiety 
as to any damages that may be entailed. Fresh air, pure air, 
will always be accessible to us all whatever we may do to impair 
it, and from the most crowded haunts of men we need never go 
many miles to find it, and it is ''free as the air " 



CHAPTER II 
DEFINITIONS AND GENERAL INFORMATION 

As we here aim to provide handy rather than minutely precise 
information for those generally who may have to do with air for 
mechanical and practical uses, and as we are not proposing to 
add to the stock of facts and data of the expert scientist, the 
common English-American standards of weight and measurement 
will be employed. To have it at hand for immediate use when 
required, an international conversion table of sufficient scope is 
given on page 10. 

Referring to this table, the figures in the column following 
that containing the names of the French units are to be used as 
multipliers in converting those units into their equivalents in 
English or American measures. Thus 17 meters multipUed by 
39.37 equals 669 in. The figures in the column of reciprocals 
are similarly to be used as multipliers for converting English 
measures into their French equivalents. Thus 25 sq. in. multi- 
plied by 6.452 equals 161.3 sq. cm. 

All measures of length or distance which occur in the book will 
be given in feet and inches, and weights in pounds avoirdupois. 
When air, steam or other pressures are referred to they will be 
pressures in pounds per square inch as indicated upon a common 
pressure gage, or pressures above that of the normal atmosphere. 
Absolute pressure is the gage pressure plus the pressure of the 
atmosphere at the given time and place, this atmospheric pressure 
being usually taken as 14.7 lb. at sea level. It would be more 
convenient and would lead to closer approximations in our 
guess-work calculations if 14.5 were used instead. 

Table II., of various pressure equivalents, requires no explan- 
ation. It is used in publications of the General Electric 
Company. 

For temperatures the Fahrenheit scale will be used ex- 
clusively. A simple formula for converting Centigrade read- 
ings to Fahrenheit is: 

9/5C-h32=F. 
9 



% 



10 



COMPRESSED AIR PRACTICE 



TABLE I 
INTERNATIONAL CONVERSION TABLE OF WEIGHTS AND MEASURES 



French units 



Millimeters 

Centimeters 

Meters 

Meters 

Meters 

Kilometers 

Kilometers 

Kilometers 

Sq. millimeters. . . 
Sq. centimeters. . 

Sq. meters 

Sq. meters 

Sq. kilometers. . . 
Sq. kilometers. . . 

Hectares 

Cu. centimeters. . 

Cu. meters 

Cu. meters 

Centiliters 

Liters 

Liters 

Liters 

Liters 

Liters 

Hectoliters 

Hectoliters 

Hectoliters 

Hectoliters 

Grams 

Grams water .... 

Grams 

Kilograms 

Kilograms 

Kilograms 

Kilograms per sq. 

centimeter. 
Kilogrammeters . . 
Kilos per cheval . 

Kilowatts 

Joules 

Calories 



Multiplier 






39 
3 
1 
3280 
1093 




10 
1 
247 

2 


35 
1 


61 


33 
1 

3 


26 
2 



.03937 

.3937 

.37 

.28083 

.0936 

.83 

.61 

.62137 

.00155 

.155 

.764 

.196 

.11 

.386109 

.4711 

.0610 

.315 

.308 

.338 

.023 

.03531 

.84 

.0567 

.2642 

.531 

.131 

.42 

.84 



15.432 

0.03381 

0.03527 

35.3 

2.2046 

0.00090719 
14.2226 

7.233 
2.235 

1.34 

0.7373 

3.968 



Reciprocal 

25.4 

2.54 

0.0254 

0.3048 

0.9144 

0.0003048 

0.0009144 

1.60935 
645.2 

6.452 

0.0929 

0.836 

0.004045 

2.59 

0.4047 
16.3934 

0.02831 
0.7645 

2.9585 

0.01638 
28.316 

0.0895 

0.94636 

3.7854 

0.28316 

7.6335 

0.037854 

0.3521 

0.0647989 
29.57 
28.3495 

0.028349 

0.4536 
1102.3 

0.0703 

0.1382 
0.4474 

0.746 

1.3427 

0.25201 



English and 
American units 



Inches. 
Inches. 
Inches. 
Feet. 
Yards. 
Feet. 
Yards. 
Miles. 
Sq. inches. . 
Sq. inches. 
Sq. feet. 
Sq. yards. 
Acres. 
Sq. miles. 
Acres. 
Cu. inches. 
Cu. feet. 
Cu. yards. 
Fluid ounces. 
Cu. inches. 
Cu. feet. 
Fluid ounces. 
Quarts. 
Gallons. 
Cu. feet. 
Cu. yards. 
Gallons (231 cu. in.) 
Bushels (2150.42 cu. 
in.). 
Grains. 
Fluid ounces. 
Oz. avoirdupois. 
Oz. avoirdupois. 
Pounds. 

Ton (2000 pounds). 
Pounds per sq. in. 

Foot-pounds. 
Pounds per horse- 
power. 
Horse-power. 
Foot-pounds. 
B. t. u. 



DEFINITIONS AND GENERAL INFORMATION 11 



TABLE II 
VARIOUS PRESSURE EQUIVALENTS 



Inches water 


Inches 


Ounce per 


Pounds per 


Pounds per 


pressure 


mercury 


square inch 


square inch 


square foot 


1.00 


0.0736 


0.577 


0.036 


5.19 


13.6 


1.00 


7.84 


0.49 


70.6 


1.73 


0.127 


1.00 


0.0625 


9.0 


27.7 


2.04 


16.0 


1.00 


144.0 


0.192 


0.0142 


0.111 


0.00694 


1.00 



What will 85° Centigrade be by the Fahrenheit scale? 

(85X9)^5 = 153 and 153+32=185 
For converting Fahrenheit to Centigrade the formula is: 

5/9(F-32)=C. 
What is the Centigrade equivalent of 500° Fahrenheit? 
500-32 = 468, 468X5 = 2340, 2340^9 = 260 

On page 12 is an excellent and most convenient table (Table III) 
of Centigrade and Fahrenheit readings arranged by Dr. Leonard 
Waldo, 49 Wall Street, New York, and here reproduced from 
Metallurgical and Chemical Engineering. 

Where tables, rules or formulas are given in which numerical 
multipHers occur, the reciprocals of the multipHers are frequently 
given also, in parentheses or otherwise, as when the operations 
indicated are reversed these reciprocals can be used as multipHers, 
multiphcation being generally an easier process than long 
division. 

Atmospheric air is the vapor of a composite liquid which boils 
and evaporates completely at an extremely low temperature, 
the vapor having properties and characteristics quite analogous 
in many particulars to those of steam. While we are quite 
familiar with water as a solid, as a liquid, and as a vapor or gas, 
it is only the vapor of liquid air that we normally know about. 

Air is roughly stated to be composed of 23 parts by weight of 
oxygen and 77 parts of nitrogen. By volume the proportions are 
21 parts of oxygen and 79 parts of nitrogen. It thus appears 
that oxygen is somewhat heavier than air while nitrogen is a 
little lighter, the specific gravity of the former when separated 
being 1.106 and that of the latter, 0.974, air being 1, and when 



12 



COMPRESSED AIR PRACTICE 



TABLE III 
CENTIGRADE AND FAHRENHEIT SCALES OF TEMPERATURE 

















1 
2 
3 


F« 

1.8 
3.6 
5.4 


c° 





10 


20 


30 


40 50 1 60 


70 


80 


PO 


— 200 

— lOO 
— 


F 

-328 

-148 

+ 32 


F 

-346 

-166 

+ 14 


F 

-364 

-184 

-4 


F 

-382 

-202 

-22 


F 

-400 

-220 

-40 


F 

-418 

-238 

-58 


F 

-436 

-256 

-76 


F 

-454 

-274 

-94 


F 


F 


-292 
-112 


-310 
-130 





32 


50 


68 


86 


104 


122 


140 


158 


176 


194 


lOO 
200 

300 


212 
392 
572 


230 
410 
590 


248 
428 
608 


266 
446 
626 


284 
464 
644 


302 
482 
662 


320 
500 
680 


338 
518 
698 


356 
536 
716 


374 
554 
734 


400 
500 
6oo 


752 

932 

1112 


770 

950 

1130 


788 

968 

1148 


806 

986 

1166 


824 
1004 
1184 


842 
1022 
1202 


860 
1040 
1220 


878 
1058 
1238 


896 
1076 
1256 


914 
1094 
1274 


4 
5 
6 


7.2 

9.0 

10.8 


700 
8oo 
poo 


1292 
1472 
1652 


1310 
1490 
1670 


1328 
1508 
1688 


1346 
1526 
1706 


1364 
1544 
1724 


1382 
1562 
1742 


1400 
1580 
1760 


1418 
1598 
1778 


1436 
1616 
1796 


1454 
1634 
1814 


7 

8 

9 

10 


12.6 
14.4 
16.2 
18.0 


I coo 


1832 


1850 


1868 


1886 


1904 


1922 


1940 


1958 


1976 


1994 


IIOO 
I200 
1300 


2012 
2192 
2372 


2030 
2210 
2390 


2048 
2228 
2408 


2066 
2246 
2426 


2084 
2264 
2444 


2102 

2282 
2462 


2120 
2300 
2480 


2138 
2318 
2498 


2156 
2336 
2516 


2174 
2354 
2534 


1400 
1500 
1600 


2552 
2732 
2912 


2570 
2750 
2930 


2588 
2768 
2948 


2606 
2786 
2966 


2624 
2804 
2984 


2642 
2822 
3002 


2660 
2840 
3020 


2678 
2858 
3038 


2696 
2876 
3056 


2714 
2894 
3074 


F° 


C° 


1700 
1800 
1900 


3092 
3272 
3452 


3110 
3290 
3470 


3128 
3308 
3488 


3146 
3326 
3506 


3164 
3344 
3524 


3182 
3362 
3542 


3200 
3380 
3560 


3218 
3398 
3578 


3236 
3416 
3596 


3254 
3434 
3614 


1 
2 
3 


0.56 
1.11 
1.67 


2000 


3632 


3650 


3668 


3686 


3704 


3722 


3740 


3758 


3776 


3794 


4 
5 


2.22 
2.78 


2100 
2200 
2300 


3812 
3992 
4172 


3830 
4010 
4190 


3848 
4028 
4208 


3866 
4046 
4226 


3884 
4064 
4244 


3902 
4082 
4262 


3920 
4100 
4280 


3938 
4118 
4298 


3956 
4136 
4316 


3974 
4154 
4334 


6 

7 
8 
9 


3.33 

3.89 
4.44 
5 00 


2400 
2500 
2600 


4352 
4532 
4712 


4370 
4550 
4730 


4388 
4568 
4748 


4406 
4586 
4766 


4424 
4604 
4784 


4442 
4622 
4802 


4460 
4640 
4820 


4478 
4658 
4838 


4496 
4676 
4856 


4514 
4694 
4874 


10 
11 
12 


5.56 
6.11 
6 67 


2700 
2800 
2900 


4892 
5072 
5252 


4910 
5090 
5270 


4928 
5108 
5288 


4946 
5126 
5306 


4964 
5144 
5324 


4982 
5162 
5342 


5000 
5180 
5360 


5018 
5198 
5378 


5036 
5216 
5396 


5054 
5234 
5414 


13 
14 
15 
16 
17 
18 


7.22 
7.78 
8.33 
8.89 
9.44 
10.00 


3000 


5432 


5450 


5468 


5486 


5504 


5522 


5540 


5558 


5576 


5594 


3100 
3200 
3300 


5612 
5792 
5972 


5630 
5810 
5990 


5648 
5828 
6008 


5666 
5846 
6026 


5684 
5864 
6044 


5702 
5882 
6062 


5720 
5900 
6080 


5738 
5918 
6098 


5756 
5936 
6116 


5774 
5954 
6134 


3400 
3500 
3600 


6152 
6332 
6512 


6170 
6350 
6530 


6188 
6368 
6548 


6206 
6386 
6566 


6224 
6404 
6584 


6242 
6422 
6602 


6260 
6440 
6620 


6278 
6458 
6638 


6296 
6476 
6656 


6314 
6494 
6674 






3700 
3800 
3900 


6692 
6872 
7052 


6710 
6890 
7070 


6728 
6908 
7088 


6746 
6926 
7106 


6764 
6944 
7124 


6782 
6962 
7142 


6800 
6998 
7160 


6818 
6998 
7178 


6836 
7016 
7196 


6854 
7034 
7214 


C = 185 


2».78C 


C° 





10 


20 


30 


40 


SO 


60 


70 


80 


90 


Exampl 


28: 13^ 


17°C = 2444°F + 12° 


6F = 2456°.6F:336 


7°F = 


1850°C 


+2°.78 



DEFINITIONS AND GENERAL INFORMATION 13 

liquid air is evaporated the nitrogen boils away first, which is 
taken advantage of for the commercial segregation of these gases. 

Although the combination of oxygen and nitrogen in the air is 
a mechanical rather than a chemical one, the two gases separating 
quite easily under certain conditions, and without the interven- 
tion of other elements, and although in the life processes of both 
animals and plants these individual gases are constantly being 
abstracted in varying proportions, the composition of the atmos- 
phere as a whole is maintained with wonderful precision all over 
the world ; but it must be remembered that there are processes of 
restoration as well as of abstraction constantly in operation. 

Although oxygen comprises less than one-quarter of the atmos- 
phere it is, or has been, more studied and written about and has 
been considered of much more use and importance than the larger 
constituent. It would appear that the functions of nitrogen 
have not been fully and clearly understood, and that, now that 
attention is called to its importance in the reciprocal economics 
of animal and vegetable life, it is coming into its own in the minds 
of men. 

The simple conception of the air as composed of oxygen and 
nitrogen is inadequate and unsatisfying in many respects, because 
one cannot avoid the knowledge that the air carries with it or 
in it other ingredients, many and diverse. These contents of it, 
however, are not generally to be considered as integral parts of 
it, as they are constantly changing. The most important detail 
of the air burden is water, sometimes as vapor closely intermingled 
with and uniformly distributed through it and invisible, and 
sometimes in minute particles of the same vapor condensed into 
actual globules of liquid, and seen as clouds if at a distance or as 
fog or mist if close at hand. 

The term ''free" air, as distinguished from air which has been 
artificially compressed, is only used as a matter of convenience 
and custom. Free air, or air at atmospheric pressure, as was 
remarked in the preceding chapter, is really compressed air, or 
air subjected to pressure, as truly as air at 100 lb. pressure is com- 
pressed air, and its volume, pressure and temperature vary in 
accordance with the same laws. By free air, as the term is 
commonly used, is meant air at atmospheric pressure, and at 
ordinary temperature, and it is the air as we obtan it when we 
begin the operation of mechanical air compression. It is free 
air, or it should be free air, when first admitted to the compressor 






14 COMPRESSED AIR PRACTICE 

cylinder, and it is not free air again until it has been compressed, 
has done its work, generally in the act of re-expansion, and has 
been exhausted or discharged into the atmosphere, and has 
become again a part of the mass which encircles the earth. 

When we speak of ''free" air entering the compressor cylinder 
we do not by that term fix it in any of its varying characteristics. 
We know nothing precisely as to its pressure, its volume as related 
to its weight, its temperature or its humidity The pressure and 
volimie of a given weight or actual quantity of free air may vary 
with the altitude or location, or with the barometric reading at 
the place and time; or, again, the volume may vary with the 
temperature. The temperature may vary with the changes of 
the seasons, with the time of day, or with the general surround- 
ings. For present purposes it is herein generally assumed that 
our free air is at the normal sea-level pressure of 14.7 lb., absolute, 
and at a temperature of 60°. 

The temperatures dealt with will usually be the sensible 
temperatures, or those indicated by the Fahrenheit thermometer, 
32° being the melting-point of ice, or the point where water 
changes from the solid to the Uquid state, and 212°, or 180° above 
this, being where the change from the liquid to the gaseous 
state occurs. The boiling-point is in practice a constantly vari- 
able one, and depends entirely upon the pressure of the atmos- 
phere upon the water, 212° being the boiling-point only at ordi- 
nary atmospheric pressure near the sea level. Water may theo- 
retically be made to boil at any temperature above the freezing- 
point by sufficiently reducing the atmospheric pressure upon its 
surface. 

Absolute temperature by the Fahrenheit scale is the tempera- 
ture as indicated by the thermometer plus 461°. Thus at 60° by 
the thermometer the absolute temperature is 60+461 = 521. 
At zero temperature by the thermometer the absolute tempera- 
ture is 0+461=461. If the thermometric temperature is, say, 
30° below zero, or —30, the absolute temperature will be —30+ 
461=431, and so on. 

In all questions relating to the volume, pressure or weight of 
air, whether ''compressed" or not, the absolute temperature is 
an important factor, as the volume of the air will vary directly 
as the absolute temperature, and the pressure and the actual 
weight of the air will have changing relations. If the absolute 
temperature of any body of air is increased, the volume will be 



DEFINITIONS AND GENERAL INFORMATION 15 

increased in the same proportion, the pressure remaining constant. 
So if the absolute temperature of the air be reduced the volume 
will be reduced equally with it if the pressure is unchanged 

The relations of volume, pressure and temperature of air are 
thus summarized: 

1. The absolute pressure of air varies inversely as the volume 
when the temperature is constant. 

2. The air volume varies inversely as the absolute pressure 
when the temperature is constant. 

3. The absolute pressure varies directly as the absolute tem- 
perature when the volume is constant. 

4. The volume varies as the absolute temperature when the 
pressure is constant. 

5. The product of the absolute pressure and the volume is 
proportional to the absolute temperature. 

A cubic foot of dry air at atmospheric pressure and at any 
absolute temperature, Fahrenheit, will weigh 39.819 lb. divided 
by the absolute temperature. Thus at 60° a cubic foot of air 
weighs 39.819 ^ (60+461) = 0.0764 lb. So, inversely, the volume 
of, say, 1 lb. of air at atmospheric pressure and at any absolute 
temperature may be ascertained by dividing the temperature by 
39.819, or by multiplying by its reciprocal, 0.025114. 

Thus at 60°, as before, 521 -^39.819 = 13.084 cu. ft. or 521 X 
0.025114 = 13.084 cu. ft. 

Table IV s'lows the weight and volume of atmospheric air at 
sea-level at different temperatures, and requires no explanation. 
It will be noticed that the figures of column 3 are the reciprocals 
of those in column 2, and vice versa. 

If the temperature and the pressure of air both vary, the con- 
stant 2.7093 (reciprocal 0.3691) multiplied by the absolute 
pressure in pounds, per square inch and divided by the absolute 
temperature will give the weight of a cubic foot. 

What will be the weight of 1 cu. ft. of air at 60 lb. pressure and 
100° temperature? 

2.7093 X (60+ 14.7) -^ (100+461) =0.3607 lb. 

The volume in cubic feet of 1 lb. of air may be ascertained by 
dividing the absolute temperature by the absolute pressure and 
either dividing it by the constant 2.7093, or, preferably, multi- 
plying it by the reciprocal 0.3091. 




16 COMPRESSED AIR PRACTICE 

TABLE IV.— WEIGHT AND VOLUME OF AIR AT SEA-LEVEL AND AT 
DIFFERENT TEMPERATURES 



Temperature, 


Weight of 1 cu. ft. 


. Volume of 1 lb. in 


degrees Fahr. 


in pounds 


cubic feet 





0.0863 


11.582 


10 


0.0845 


11.834 


20 


. ... 0.0827 


12.085 


30 


0.0811 


12.336 


32 


0.0807 


12.386 


40 


. ... 0.0794 


12.587 


50 


. ... 0.0779 


1 12.838 


60 


. ... 0.0764 


13.089 


70 


0.0750 


13.340 


80 


0.0736 


13.592 


90 


0.0722 


13.843 


100 


0.0710 


14.094 


110 


0.0697 


14.345 


120 


0.0685 


14.596 


130 


. ... 0.0674 


14.847 


140 


0.0662 


15.098 


150 


0.0651 


1 15.350 


160 


0.0641 


i 15.601 


170 


0.0631 


15.852 


180 


.... 0.0621 


16.103 


190 


0.0612 


16.354 


200 


.... 0.0602 


16.605 


210 


0.0593 


16.856 


212 


0.0591 


16.907 



What will be the volume of 1 lb. of air at 75° temperature and 
50-lb. gage pressure? 

(75+461)^ (50+14.7) X0.3691 =3.0577 cu. ft. 

If the temperature of air is changed from one absolute tempera- 
ture T to another absolute temperature t, the volume remaining 
constant, the resulting absolute pressure p may be obtained from 
the original absolute pressure P by the simple proportion : 

PX t 
T:t : :P:p, or, -^- = 7?. 

If the air enclosed in an air receiver is at 50-lb. gage, or 64.7 
absolute pressure, and at 60°+461=521 absolute temperature, 



DEFINITIONS AND GENERAL INFORMATION 17 

what will be its absolute pressure if its temperature is raised to 
200+461 = 661° absolute temperature? 

(64.7X661)^521=82.08 lb. abs. 

If the temperature of air is changed from one absolute tempera- 
ture T to another absolute temperature t, as before, the pressure 
in this case remaining constant, the resulting volume v (say in 
cubic feet) may be obtained from the original volume F by the 
simple proportion : 

T:t::V:v,or ^^-=v 

If 100 cu. ft. of air at 60+461 = 521° absolute temperature 
have its temperature raised to 220+461 = 681°, absolute, what 
will then be its volume? 

(100X681) -^521 = 130.71 cu. ft. 

Specific Heat. — The unit of heat generally employed in records 
and computations is that quantity of heat which will raise the 
temperature of 1 lb. of water 1° F., this being known as the British 
thermal unit (B.t.u.). One unit of heat if apphed to 1 lb. of 
anything else will not have precisely the same heating effect 
which it has when applied to water. More heat is required to 
raise the temperature of 1 lb. of water 1° than is required for any 
other substance. The heating effect of a unit of heat apphed to 
different substances is found to vary widely, and the special 
quantity of heat required to raise the temperature of 1 lb. of 
any substance 1° is known as its specific heat. The specific 
heat of water being 1, all other specific heats are necessarily 
fractions. That of air being 0.2377, or less than one-quarter 
that of water, the same unit of heat which would raise the 
temperature of 1 lb. of water 1° would raise the temperature of 
1 lb. of air more than 4°. 

The addition of heat to air, or to any elastic fluid may have 
either of two effects. It may increase the volume while the pres- 
sure remains constant, or it may increase the pressure while the 
volume remains constant. The specific heat, however, will be 
quite different in the two cases. The specific heat of air — 0.2377, 
as given above — is its specific heat at constant pressure, and the 
heat in this case exhibits its effect by increasing the volume of 
the air. If the air be confined so that there can be no increase of 



18 COMPRESSED AIR PRACTICE 

volume, its specific heat is then only 0.1688, or about one-sixth 
that of water. If heat be applied to air under constant pressure, 
raising its temperature from the freezing- to the boiling-point of 
water — from 32° to 212° — the increase in volume will be from 
1 to 1.365; and if heat be applied to air at constant volume, 
raising its temperature as before from 32° to 212°, the increase 
in absolute pressure will be from 1 to 1.365, the numerical result 
being alike in the two cases, but the heat expended will be as 
0.2377:0.1688, or nearly one-half more in one case than in the 
other. 

When air is compressed, or when its volume is reduced by the 
application of force, the temperature of the air is raised. This 
phenomenon occurs entirely regardless of the time occupied in 
the compression; the heating does not follow the compression 
but is coincident with it. If during the compression the air 
neither loses nor gains any heat by conduction or radiation to or 
from any other body, the heat produced by the act of compression 
remaining in the air and increasing its temperature, then the air 
is said to be compressed adiabatically, and such compression is 
adiabatic compression. 

When air under pressure is allowed to expand into a larger 
volume its pressure and its temperature both fall, and if during 
the operation the air receives no heat from anything outside 
itself, it is said to expand adiabatically. Adiabatic compression 
or expansion of air is compression or expansion without the air 
losing or gaining any heat. The expression '^ without loss or gain 
of heat," it will be understood, does not mean the maintaining of 
the air at a constant temperature, but precisely the reverse of 
that. 

If during compression the air could be kept at a constant tem- 
perature, by the abstraction of the heat as fast as it was developed, 
the air would then be said to be compressed isothermally. In 
isothermal compression or expansion the air remains at a constant 
temperature, and it therefore must and does lose or gain heat 
from some source outside itself throughout the operation. 

The rate of increase in the temperature of air during adiabatic 
compression is not uniform. The temperature rises faster at the 
beginning and during the earlier stages of the compression than 
it does when the higher pressures are reached. Thus in compress- 
ing air from a pressure of 1 atmosphere to 2 atmospheres by 
mechanically reducing its volume, the increase in temperature 



DEFINITIONS AND GENERAL INFORMATION 19 

will be greater than in compressing from 2 to 3 atmospheres, 
and so on. The rate of increase of temperatm-e also varies with 
the initial temperature. The higher the initial temperature the 
greater will be the rate of increase of temperature at any point, 
and throughout the subsequent compression. 

The Barometer. — Nature is a compressed-air worker. The 
routine of natural activities includes, among many other things, 
the constant and apparently irregular changing of the air- 
pressure, or the compression and the re-expansion of the air which 
encloses us. The crudeness, the wastefulness, and the com- 
parative inefficiency of man's methods are indicated by the range 
of pressures which he is compelled to employ in his manipula- 
tions of the air, as compared with the local ranges of atmospheric 
pressures. 

In our industrial operations we have need of pressure gages 
with an aggregate reach of at least 3000 lb. to the square inch, 
while practically the entire range of pressures of the compressed 
air which nature works with in any special locality on the earth 
is only about a pound, or, say, at sea-level from 29 to 31 in. of 
mercury. The usual changes for a week at a time may all be 
within a quarter of a pound of pressure, or say half an inch of 
mercury, and yet we find it pays to watch and to keep a record of 
these minute changes, for they go along with the water-carrying 
and water-distributing functions of the air; and the effects of 
these, the rain, the fog and then the sunshine, it is desirable to 
anticipate and to make the most of for our good. 

The atmospheric pressure changes being so small, it is necessary 
to be able to measure them with great minuteness, and so we 
have the barometer, which is simply a pressure gage, but with 
some wonderful refinements which give minute precision of 
record, as spoken of in the preceding chapter. 

The diminution of the pressure of air, or its attenuation as 
altitude increases, makes it necessary to allow for this in deter- 
mining the dimensions and capacities of air compressors when 
definite amounts of work are required, and Table V will be 
of service for this purpose. It is designed to cover the entire 
range of atmospheric pressures likely to be encountered within 
the working altitudes, not only above sea-level but for 1000 ft. 
below it. The table includes for each altitude the height of the 
mercury column, the corresponding absolute air-pressure in 
pounds per square inch, the boiling-point of water, the weight of 



20 



COMPRESSED AIR PRACTICE 



a cubic foot of free air, the percentage of weight of this compared 
with sea-level air and the number of cubic feet of free air which 
at each altitude would weigh 1 lb. 



TABLE V. 



28.90 
28.69 
28.49 
28.28 
28.08 

27.88 
27.67 
27.47 
27.27 
27.06 

26.86 
26.66 
26.45 
26.25 
26.05 

25.84 
25.64 
25.44 
25.23 
25.03 

24.83 
24.62 
24.42 
24.22 
24.01 



-VOLUME AND WEIGHT OF ATMOSPHERIC AIR AT DIFFERENT 
ALTITUDES 



12.65 

12.74 
12.82 
12.91 
12.99 

13.09 
13.17 
13.26 
13.36 
13.45 

13.55 
13.64 
13.74 
13.84 
13.94 

14.04 
14.14 
14.25 
14.35 
14.46 




14.2 
14.1 
14.0 
13.9 
13.8 

13.7 
13.6 
13.5 
13.4 
13.3 

13.2 
13.1 
13.0 
12.9 

12.8 

12.7 
12.6 
12.5 
12.4 
12.3 

12.2 
12.1 
12.0 
11.9 
11.8 



i .07640 

' .07589 

.07536 

211 .07484 
07432 



1,000 
1,200 
1,400 
1,600 
1,800 

2,000 
2,100 
2,300 
2,500 
2,700 

2,900 
3,100 
3,300 
3,500 
3,700 

4,000 
4,200 
4,400 
4,600 
4,800 

5,000 
5,200 
5,400 
5,600 
5,800 



210 
209 

208 
207 

206 

205 
204 

203 
202 



.07380 
.07329 
,07277 
,07225 
,07173 

,07120 
,07068 
07016 
, 06965 
,06913 



100.00 
99.33 
98.63 
97.95 
97.27 

96.59 
95.93 
95.25 
94.96 
93.89 

93.19 
92.51 
91.83 
91.16 
90.48 



06861 


89.81 


14.57 


06809 


89.12 


14.68 


06757 


88.44 


14.79 


06705 


87.76 


14.91 


06652 


87.07 


15.03 


06600 


86.38 


15.15 


06549 


85.72 


15.27 


06497 


85.04 


15.39 


06445 


84.36 


15.51 


06393 


83.67 


15.64 


06341 


83.00 


15.77 


06289 


82.31 


15.90 


06237 


81.63 


16.03 


06185 


80.95 


16.16 


06133 


80.23 


16.30 



DEFINITIONS AND GENERAL INFORMATION 21 

TABLE v.— VOLUME AND WEIGHT OF ATMOSPHERIC AIR AT DIFFERENT 
ALTITUDES.— (Con/inued) 



Barom- 


Absolute 


Altitude 


Boiling- 


Weight 
1 cu. ft. 


Percentage 
of sea-level 


Cubic 
feet per 


eter 


pressure 




point 


eo'' 


weight 


pound 


23.81 


11.7 


6,100 


201 


.06081 


79.59 


16.44 


23.60 


11.6 


6,300 




.06029 


78.91 


16.58 


23.40 


11.5 


6,500 


200 


.05977 


78.23 


16.73 


23.20 


11.4 


6,800 




.05925 


77.55 


16.87 


22.99 


11.3 


7,100 


199 


.05873 


76.87 


17.02 


22.79 


11.2 


7,300 





.05821 


76.19 


17.18 


22.59 


11.1 


7,600 




.05769 


75.51 


17.33 


22.38 


11.0 


7,900 


198 


.05717 


74.83 


17.48 


22.18 


10.9 


8,100 




.05665 


74.15 


17.65 


21.98 


10.8 


8,400 


197 


.05613 


73.47 


17.81 


21.77 


10.7 


8,600 




.05561 


72.79 


17.98 


21.57 


10.6 


8,900 


196 


.05509 


72.11 


18.15 


21.37 


10.5 


9,100 




. 05457 


71.42 


18.32 


21.16 


10.4 


9,400 


195 


.05405 


70.74 


18.50 


20.96 


10.3 


9,600 





.05353 


70.06 


18.68 


20.76 


10.2 


9,900 




.05301 


69.38 


18.86 


20.55 


10.1 


10,100 


194 


.05249 


68.70 


19.05 


20.35 


10.0 


10,400 




.05198 


68.03 


19.24 


20.15 


9.9 


10,700 


193 


.05146 


67.35 


19.43 


19.94 


9.8 


11,000 




.05084 


66.67 


19.63 


19.74 


9.7 


11,200 


192 


.05041 


65.98 


19.83 


19.53 


9.6 


11,500 




.04990 


65.31 


20.04 


19.33 


9.5 


11,800 


191 


.04937 


64.49 


20.25 


19.13 


9.4 


12,100 




.04886 


63.95 


20.46 


18.93 


9.3 


12,400 


190 


.04834 


63.27 


20.68 


18.72 


9.2 


12,700 





. 04782 


62.59 


20.91 


18.52 


9.1 


13,000 


189 


.04730 


61.91 


21.14 


18.31 


9.0 


13,400 


188.5 


.04678 


61.23 


21.37 



The Mercury Gage and Its Successors. — In the beginning of 
modern steam-engine practice, when working steam pressures 
were very low, and when there was a condenser as an integral part 
of each engine, it became necessary for the engineer to know both 
his steam pressure and the tenuity or otherwise of his vacuum, 
and so steam gages and vacuum gages came to be used. For the 
want of better means the mercury column was used almost 
universally both for the steam pressure and for the vacuum. 

3 



I 



22 COMPRESSED AIR PRACTICE 

Within the memory of the present writer steamboats were 
running upon American waters which had open mercury steam 
gages. The mercury was contained in an inverted siphon 
composed of iron pipe. One of the vertical pipe ends was con- 
nected with the boiler and in the other or open end, which was 
longer, there was a loosely sliding wooden rod the end of which 
floated upon the mercury column, and the pressure was indicated 
by the height of this rod. A similar siphon was used for the 
vacuum gage, the long end in this case being connected with the 
condenser and the other end having the floating stick in it the 
same as the steam gage. 

This twofold use of the mercury column developed an anomaly 
in practice which has survived to this day. For the steam-pres- 
sure gage the reading began at zero for normal atmospheric 
pressure, and as the pressure increased and the rod floating upon 
the mercury rose, its increasing height above the zero mark was 
recorded as the steam pressure. For 10 lb. of steam pressure 
above the atmosphere there would be about 20 in. of mercury, 
and so on. 

For the vacuum gage, however, starting from zero as before, 
the scale for the mercury column was made to read increasingly 
as the pressure decreased. Thus from zero, or an absolute pres- 
sure »f, say, 15 lb., which was near enough for those days, if 
the absolute pressure was reduced by vacuum say, 51b., the reading 
of the mercury gage would not be 5 lb. minus, but 5 lb. plus. 
We still use the mercury values for our vacuum gages, although 
we no longer use the mercury, and we also continue to read the 
gage record inversely. 

The use of mercury for either steam or vacuum gages is now 
practically abandoned, while the readings on the dials of our 
vacuum gages are still given in inches of mercury. It would 
seem to be desirable that the equivalents in pounds per square 
inch should also be given, or, better still, the pounds only. 
Although the pounds indicated would be pounds of vacuum, or 
of diminution of pressure, this would be the most convenient in 
steam-engine practice, as the amount of reduction of pressure 
by any partial vacuum would then be directly added to the steam 
pressure on the other side of the piston to give the total effective 
pressure. 

Compressed Air by the Pound. — In statements and computa- 
tions having to do with the compression, transmission and use 



i 



DEFINITIONS AND GENERAL INFORMATION 23 

of air the basis or unit of quantity is the volume of free air, 
usually in cubic feet, or air at atmospheric pressure at the time 
and place under consideration. There are some inconveniences 
and uncertainties about this practice, because the value of the 
unit is constantly varying with both the altitude, or normal local 
pressure, and with the temperature. The actual working volume 
can always be compared with what would be the volume at sea- 
level and at an accepted mean temperature, say 60°, but it is an 
unwelcome operation. 

To know the actual quantity of air handled at any time and 
place wouldTbe to work with more certainty. If the actual 



■° 0.06 
O 



o 

<M 0.05 
o 



______ _ 1 T~"4 




■~ ^ 


"^^ 


■«. ^ 


" "^~. ^ 


rrrrkJ rrrrtHt^'e; 




~ ^»_ ~ ""T^iSS'^'fU/^i 


""■■^-^^W. T'^^Sfe^s,, 


■^^^ ■^->S-^'eg; r "-^au^e ,dl 


1 1 [rfUM M In^^?^^ l^Tn "^^ 




""-- "~T->5u;?^ ii^y 1 r r""T^ '""""-■- 


--: ^^ T-- "75(fiTtif'-4I^-- -""^T"^ " "^"^^5 


u_L[_nihtkpi^^ f^ — £p;ji^8j-rj~i ^Tn^iijji rrn mpg^ 


""""""■- "^ftT1%^^^2ep3o H=^^^-. "■^=---j^ 


::::--:==-::::: -?9¥§^5£3p=^:::::^ 


± z=|=H*Wz^ 


1 1 ==-.>____ ---y — ^~ 


1 ~ ~" — r 1- 


1 1 _l_ J_ 



20 40 m 80 100 120 140 160 180 200 212 

Temperature, Degrees Fahrenheit 

Fig. 2.— Weight of Free Air at Various Altitudes and Temperatures. 

weight of the air is known it remains the same whatever the 
transformations of condition. Pressure, volume, and tempera- 
ture may all vary, but "a pound's a pound for a' that." 

To facilitate the using of the weight basis for records and com- 
putations, Table VI and the diagram Fig. 2 have been prepared 
Computing the items of this table was in detail a simple opera- 
tion. The weight of a cubic foot of air, whether ''compressed" 
or not, is obtained, as previously explained, by multiplying the 
constant 2.7093 by the absolute pressure in pounds and dividing 
the product by the absolute temperature. Thus the weight of 
1 cu. ft. of air at an altitude of 10,000 ft. (absolute pressure 10.04 
lb ) at 80° is: 



24 



COMPRESSED AIR PRACTICE 



The diagram Fig. 2 embodies all the data comprised in the 
table, while the curve at the right hand, where it cuts the alti- 
tude curves, gives the boiling-points of water at these altitudes. 

TABLE VI.— WEIGHT OF 1 CU. FT. OF FREE AIR AT VARIOUS ALTITUDES 
AND TEMPERATURES 





Sea-level 2500 ft. 


5000 ft. 


7500 ft. 


10,000 ft. 12,500 ft. 


15,000 ft. 


Temp. 


abs. 


abs. 


abs. 


abs. 


abs. 


abs. 


abs. 


Fahr. 


press. 


press. 


press. 


press. 


press. 


press. 


press. 




14.72 


13.34 


12.14 


11.84 


10.04 


9.11 


8.29 





0.0863 


0.0785 


0.0713 


0.0648 


0.0589 


0.0535 


0.0487 


10 


0.0845 


0.0768 


0.0698 


0.0635 


0.0577 


0.0524 


0.0477 


20 


0.0827 


0.0752 


0.0683 


0.0622 


0.0565 


0.0513 


0.0467 


30 


0.0811 


0.0737 


0.0669 


0.0609 


0.0554 


0.0503 


0.0457 


32 


0.0807 


0.0734 


0.0667 


0.0606 


0.0551 


0.0501 


0.0455 


40 


0.0794 


0.0722 


0.0656 


0.0596 


0.0542 


0.0493 


0.0448 


50 


0.0779 


0.0708 


0.0643 


0.0585 


0.0532 


0.0483 


0.0439 


60 


0.0764 


0.0695 


0.0631 


0.0574 


0.0522 


0.0473 


0.0431 


70 


0.0750 


0.0682 


0.0619 


0.0563 


0.0512 


0.0465 


0.0423 


80 


0.0736 


0.0669 


0.0608 


0.0552 


0.0502 


0.0456 


0.0415 


90 


0.0723 


0.0657 


0.0596 


0.0542 


0.0493 


0.0448 


0.0408 


100 


0.0710 


0.0645 


0.0586 


0.0533 


0.0484 


0.0440 


0.0400 


110 


0.0697 


0.0634 


0.0578 


0.0523 


0.0476 


0.0432 


0.0393 


120 


0.0685 


0.0623 


0.0565 


0.0514 


0.0466 


0.0424 


0.0386 


130 


0.0674 


0.0612 


0.0556 


0.0506 


0.0459 


0.0417 


0.0378 


140 


0.0662 


0.0602 


0.0549 


0.0497 


0.0452 


0.0410 


0.0373 


150 


0.0651 


0.0592 


0.0538 


0.0489 


0.0444 


0.0404 


0.0367 


160 


0.0641 


0.0583 


0.0529 


0.0481 


0.0438 


0.0397 


0.0361 


170 


0.0631 


0.0573 


0.0521 


0.0473 


0.0431 


0.0391 


0.0358 


180 


0.0621 


0.0565 


0.0513 


0.0466 


0.0424 


0.0383 


0.0348 


190 


0.0612 


0.0556 


0.0504 


0.0459 


0.0417 


0.0379 


0.0343 


200 


0.0602 


0.0547 


0.0487 


0.0452 


0.0411 


0.0373 


0.0338 


210 


0.0593 


0.0539 


0.0489 


0.0445 


0.0405 


0.0368 


0.0332 


212 


0.0591 


0.0538 


0.0488 


0.0444 


0.0404 


0.0366 


0.0331 



The diagram shows at a glance the differences in weight, or 
in the actual quantity of air, comprised in a cubic foot of free air 
at the different elevations. Thus, to take the extremes, the 
weight of a cubic foot of free air at sea level and at 60° is 0.0764 
lb., while at an elevation of 15,000 ft. the weight of the same 
volume of air at the same temperature is 0.0431, this latter being 
only 56 per cent, of the former. 



I 



DEFINITIONS AND GENERAL INFORMATION 25 

In compressing this air to, say, 100 lb. local gage pressure, the 
ratio of compression at sea level would be: 

100+14.7 ^o . 1. 
— :rjij — = 7.8 atmospheres, 

while at 15,000 ft. it would be 
100+8.29 



8.29 



= 13.06 atmospheres. 



This suggests the difference in the work of compression and 
in the results of compression at different altitudes. 



CHAPTER III 



THE COMPRESSED-AIR PROBLEM 



The general problem of air compression and of the employment 
of compressed air for the transmission and redevelopment of 
power wherever it may be required can be stated in simple 
terms. The crude sketch, Fig. 4, embodies practically all the 
essentials. 

The piston F is fitted to the cylinder E so that we may assume 
it to move freely and without friction or leakage. It is also 
supposed to be without weight, and in 
speaking of it in various positions in the 
cylinder the lower face of the piston only 
is referred to. 

The piston being at A, as shown, and the 
cylinder being full of free air, or air at the 
normal pressure of 1 atmosphere, and at 
whatever may be the local temperature at 
the moment, a sufficient weight is placed 
upon the piston to force it down into the 
cyhnder and to compress the air contained 
in it to a pressure of, say, 6 atmospheres. 

The theoretical volume being inversely as 
the absolute pressure, the piston should go 
down to C, enclosing a space one-sixth of 
that which the air occupied at the beginning. 
We find, however, that the piston actually 
goes down only to B, and the reason is that 
while the air is being compressed the opera- 
tion of compression also heats the air, and the rise of temperature 
is accompanied by an increase of volume, so that the space 
which it occupies after the compression is considerably larger 
thaii it should be upon the assumption that the volume is inver- 
sely as the pressure. 

Supposing both the piston and the cylinder to be absolute 
non-conductors of heat, and that the air heated by the compres- 

26 



Fig. 4.— The Com 
pressed- air Problem. 



THE COMPRESSED-AIR PROBLEM 27 

sion loses none of this heat of compression, then if the weight 
which forced the piston down to B, be removed the piston will 
be driven back to its original position A, and the air contained 
in the cylinder will have resumed its normal volume, pressure 
and temperature, and it will have done as much work, or will 
have exerted as much force, in the expansion as was apphed to 
it in the act of compression. 

If, however, while the piston was at B, with the weight upon 
it sufficient to balance the pressure of 6 atmospheres, the air in 
some way had given off all its heat of compression and had been 
cooled to its original temperature, the piston would have de- 
scended to C, and the law that the volume varies inversely as the 
absolute pressure would have held good, for then the initial and 
the terminal temperatures would have been the same. 

We have considered thus far only the compression of the air 
and what occurs to it in consequence of its change of temperature. 
We will find that temperature also has its complicating effect 
when we proceed to use the air for motor purposes, or attempt to 
obtain through its action a return of the force which has been 
expended in its compression. 

The air being cooled to its original temperature, and the pis- 
ton being at C, we might expect that upon removing the compress- 
ing weight the piston would return to A, its original position. 
It is found, however, in practice that the piston will return only 
to D. When it reaches this point the pressure of the air in the 
cylinder has fallen to its original pressure of 1 atmosphere, and 
the piston at D is balanced between the equal pressures above and 
below it. 

As the air is heated in the operation of compression so does its 
temperature correspondingly drop when any expansion or re- 
expansion occurs, and when the rising piston reaches D the air 
in the cyfinder is already down to atmospheric pressure, because 
it is then much cooler than when its return movement upward 
began, and it is solely because of this loss of heat that the piston 
does not return to A from whence it started. 

If while the piston is at D the air can by any means recover 
all the heat which it is here assumed to have lost while com- 
pressed, the piston will return to A as before. This sketch. Fig. 
4, is drawn approximately to scale when the initial compression 
is to 6 atmospheres as here spoken of, and from it we may get 
a crude idea of the power loss entailed. The distance DA 



28 



COMPRESSED AIR PRACTICE 



compared with CA, or the distance DC, represents the total 
possible loss of power, with the pressures here assumed, in the 
compression and re-expansion of the air, theory here taking 
cognizance of all the conditions. 

The use of the weight upon the piston as above for compress- 
ing the air, and then the removal of the weight all at once when 
we wish to allow the air to re-expand is somewhat misleading. 
In the compressing of the air and in the re-expanding of the air- 
by its thrust against the piston work is being done, and the weight 
does not correctly represent the force used. In the beginning of 
the compression the weight or power required is very small, and 



T) 1 




Fig. 5. — Diagram Showing Compression and Re-expansion Effects. 

while this power requirement gradually increases it is only at the 
last moment of the compression that the total weight spoken 
of is required. It is the same inversely with the re-expansion. 
When that begins the total weight which opposes the piston is 
not all at once removed, but is gradually diminished as the pis- 
ton advances until it all disappears. 

We may now refer to the more or less practical indicator 
diagram Fig. 5, scale 40, which is intended to show more clearly 
the practical possibilities in the compression and use of air at 
75 lb., or 6 atmospheres, absolute. The line ah is the adiabatic 
compression-Une, or the line of effective resistance encountered 
in the compression stroke upon the assumption that no heat is 
taken away from or is lost by the air during the compression. 



THE COMPRESSED-AIR PROBLEM 29 

The initial temperature of the air being 60°, the final tempera- 
ture would be about 415°, and the final volume would be 0.28 
of the original volume. The line ac is the isothermal compres- 
sion-line, which assumes that all the heat of compression is 
got rid of just when it is produced, or that the air throughout 
the compression remains constantly at its initial temperature. 
The final volume in this cases is 0.1666 of the original volume. 
Remembering that these lines, ah and ac, represent the compres- 
sion of the same initial volume of air, it is evident that there 
is quite a difference in the amount of power employed in the 
two cases, and herein Hes the loss, or the possibility of loss, of 
power in the operation of compression. The mean effective 
pressure or resistance of the air for the stroke upon the adiabatic 
line ahl is 35.36 lb., while the mean effective pressure for the 
isothermal compression-line ad is but 27 lb., or only 76 per cent 
of the former. 

The comparison should, however, be reversed. The adiabatic 
mean effective pressure is 131 per cent, of the isothermal mean 
effective pressure: 

27:35.36: :1: 1.31. 

and this 31 per cent, is, of course, the additional or, as we might 
say, the unnecessary power employed, assuming isothermal 
compression to be attainable. 

Neither of these compression-lines, ah or ac, is possible in 
practice. Air cannot be compressed without losing some of its 
heat during compression, so that the actual compression-line 
must always fall within or below the fine ah. On the other 
hand, it is equally impossible to abstract all the heat from 
the air coincidently with the appearance of that heat, so that 
the actual compression-line must always fall outside or above 
the line ac. The dotted line ao represents the mean of the 
adiabatic and the isothermal fines, and the best attainable 
practice may be expected to run outside this line ao. The mean 
effective pressure for the fine aol, which includes the expulsion 
or delivery of the air in addition to its compression is about 
31 5 lb., or stiU nearly 17 per cent, in excess of the M.E.P. for 
the line act. 

In all these comparisons for efficiency the actual compression 
and delivery line is always to be compared with the isothermal 
line aclj because that is the ideal lino for compression without 



30 COMPRESSED AIR PRACTICE 

sacrifice of power, and because the terminal volume cl is the 
volume actually available for use, no matter how economically 
or how wastefully the air may have been compressed. Though 
at the completion of the compression stroke there is always some 
of the heat of compression remaining in the air, and though its 
volume at the instant of delivery is always greater than cl, 
that heat may be said to be always lost in the transmission of 
the air, or in its storage, and the available volume is never 
practically above cl. 

After the cooling and contraction of the air compressed comes 
the question of the cost of transmitting and distributing it to 
the points where it is to do its work. To cause the air to flow 
through the pipes there must be some excess of pressure at the 
first end, a decrease of pressure as the air advances and a greater 
or less reduction of effective pressure at the delivery end. But 
the ultimate loss of power in transmission has been greatly 
exaggerated. 

The actual truth is that there is very little loss of power through 
the transmission of compressed air in suitable pipes to a reason- 
able distance, and the reasonable distance is not a short one. 
With pipes of proper size, and in good condition, air may be 
transmitted, say, ten miles, with a loss of pressure of less than 
1 lb. per mile. If the air were at 80 lb. gage, or 95 lb. absolute, 
upon entering the pipe, and 70 lb. gage, or 85 lb. absolute, at 
the other end, there would be a loss of a little more than 10 per 
cent, in absolute pressure, but at the same time there would be 
an increase of volume of 11 per cent, to compensate for the loss 
of pressure, and the loss of available power would be less than 
3 per cent. With higher pressures still more favorable results 
could be shown. 

Having compressed the air and conveyed it to the point 
where we wish to use it, we may turn again to Fig. 5, and see 
what we will be able to do with the air. The air may be used 
in various ways with widely different economic results, and 
little ingenuity would be required to develop considerable 
losses, if losses were what we were after. Having the volume 
cl, and using it in a cylinder of suitable capacity, with a point 
of cut-off which would allow the air to expand down to about 
1 atmosphere before release, the adiabatic expansion line, or 
the lowest line that the air could make would be the fine cd, 
and the total loss in the use of the air, as compared with the 



THE COMPRESSED-AIR PROBLEM 31 

power cost of compressing it, would be the difference between 
the areas aolh and Icdh, the latter being 66 per cent, of the 
former. 

The temperature of the air at c, where the expansion begins, 
being assumed to be 60°, the coohng of the air which accompanies 
its expansion will bring the temperature far down the scale 
when d is reached, d being the end of the cylinder wherein the 
expansion takes place. The theoretical temperature of the air 
at the end of the stroke would be about —150°. The actual 
temperature in these cases is never found as low as the theoretical 
temperature, because the air receives heat from the cylinder 
and from the walls of the passages with which it comes in con- 
tact; but it is usually still cold enough to cause serious in- 
convenience in practice, and this cooling of the air may in many 
cases be prohibitive, entirely regardless of the economy of the 
case. The air always contains moisture, the amount varying 
with the surrounding meteorological conditions, and as the air 
becomes attenuated and so intensely cold the water is rapidly 
frozen in the passages, and soon chokes them up and stops the 
operation of the motor. The prevention or the circumvention 
of the freezing up of air apparatus is an additional complication 
of the compressed-air problem to be considered later. 

The trouble from the freezing up naturally suggests the heat- 
ing of the air before it is used. The heating or re-heating of the 
air, where it is practised, not only brings us out of our trouble 
about the freezing up, but it increases the volume of the air 
and its consequent available power at a very slight expense for 
the heating. If the volume of air cl, being now at 60°, be passed 
through a suitable heater and its temperature raised to 300°, its 
volume will then be il, instead of cl, or 0.2434 instead of 0.1666, 
an increase of volume of about 50 per cent. In practice, to in- 
sure a temperature of 300° in the cylinder at the beginning of 
the expansion; it will be necessary to heat the air considerably 
above that temperature, say to 400°, as the air loses its heat 
very rapidly. If now we use this re-heated air, the volume 
cl, becoming il, and expanding this air down to e, supposing 
the temperature at i to be 300° the final theoratical temper- 
ature will be about zero. The actual temperature, it is 
pretty certain, will not be below the freezing-point, and all 
our trouble about the freezing up of the passages will have 
disappeared, and the power realized per volume of free air 



82 COMPRESSED AIR PRACTICE 

used will have boen much incroasod. It sooms to be quite 
prnctical)lo in many cases, by effective cooling of the air 
(luriiiK its compression, and by ro-hoating it l>«foro its re- 
expansion, to bring the expansion-line ic to enclose with the ad- 
mission-line an area not less than that enclosed by the comprae- 
sion-line as, and then the losses will lx> those attributjiblo to 
the clearanc(>s and to friction, while there will l>e the slight 
additional charge for the re-heating. 



CHAPTER IV 

TABLES AND DIAGRAMS FOR COMPUTATIONS 
IN AIR-COMPRESSION 

Tho tables and diajcrams presented in this chapter should l>6 
of service to those having to do with air-compression, or with 
compress<Ml air uschI for motor i)uri><>s<'s, wlio wish to rea<*h ap- 
proximately reliable results by simple and familiar methods. 

The actual compression of air for industrial purpos<^s and by 
tho meth(xis still most Kcncrally employed, that is in cylinders 
with reciprocating pistons, is practically adiabatic in u;u-h si>ocific 
case, and in investigatinK th(? power nniuirements. the actual 
power consumption and (ither conditions or results of such 
compression, it is necessjiry to know as a basis tho pressures, 
volumes and temixTatures invoIv«Ml, and also the mean effective 
pressures or resistanctis occurring, and the facilities here pn^'nlmi 
should make tho work oasy. Tho tablos aro applicable to all 
cas<ts of single-stage fompression, aiul to all tho separate sUigL^ 
of compound- or multi-stage compression. 

When other and entirely different, ami jx^rliaps equally effi- 
cient, devices are employi^l for compressing air. such as tur)K>- 
cornpressors, hydraulic n)mpresst)rs, tac, it is still mmvenient to 
refer to the work of tho nHMpro<:ating compntssor for the compari- 
sion and valuation of results a<-complish<»<l. 

PreMures, Volumes and Temperttures. Ut'ferring to Table 
VII and its corresponding diagram. Fig. 0, it will \m m^m that if 
wo first asi'ertain the ratio of the initial to the UTminal (or 
vice veraa) of eitlutr the pressures, the V(»luint»rt or the ten»|M»ratun« 
of air oompn»KH<»d and delivennl without hnw or Kain c»f h»«t, 
adiabatio cfimpntwion, the rati«» of the other two purticulani are 
cmMily determin«H| and then having the ratio in ««itlmr ra?*». with 
the actual presHuni, volume or tiMuiMtratun* at one eml. either tho 
bojjinning or the completion, of the not of ctinipn^iMion (or of r«- 
oxpannion if the air is being uh«m1 for m- ' tlio cor- 

retf|>onding preHMunts, volumcM or t4MiiiMM iior ond 

of tho o|>eration alfiu will U) known. 



34 



COMPRESSED AIR PRACTICE 



TABLE VII. 



-RATIOS AND RECIPROCALS OF PRESSURE, VOLUME AND TEMPERA- 
TURE IN THE ADIABATIC COMPRESSION OF AIR 



tf 



P^ 



<o o 
Ph >- 



0.01 

0.02 

0.03 

0.04 

0.05 

0.06 

0.07 

0.08 

0.09 

0.10 

0.11 

0.12 

0.13 

0.14 

0.15 

0.16 

0.17 

0.18 

0.19 

0.20 

0.21 

0.22 

0.23 

0.24 

0.25 

0.26 

0.27 

0.28 

0.29 

0.30 

0.31 

0.32 

0.33 

0.34 

0.35 

0.36 

0.37 

0.38 

0.39 

0.40 

0.41 

0.42 

0.43 

0.44 

0.45 

0.46 

0.47 

0.48 

0.49 

0.50 



100.0 
50.0 
33.33 
25.0 
20.0 
16.66 



0.0380 

0.0621 

0.0829 

0.1017 

0.1191 

0.1356 

14.28 jo. 1513 

12.5 |o.l663 

11.11 !0.1808 

10.0 10.1949 

9. 091 !0. 2085 

8.333i0.2218 

7.692:0.2348 



7.143 

6.667 

6.25 

5.882 

5.556 

5.263 

5.0 

4, 

4. 

4. 

4. 

4, 

3 



315!0.0131 76.259 0.0015 666. 
103;0. 0252 39.682 0.004l|243. 
0620. 0369 27 . 102!o . 0072 138 



. 0075 133 . 33 . 1528 6 . 554 . 2633 3 . 799 



90 



83210.0483120.704 



396 0.0596} 16. 778 
374j0. 07081 14. 039 
60910.0819112.210 



0.2475 
0.2599 
0.2721 
0.2841 
0.2959 
0.3074 
0.3188 
0.3301 
0.3412 
0.3521 
0.3629 
0.3736 
0.3841 
0.3946 
3.571 0.4049 
3.44810.4151 
3.333i0.4252 
3.226|0.4353 
3.12510.4452 



762 
545 
348 
167 


846 
3.704 



3.03 
2.941 

2.857 



0.4550 

0.4648 

0.4744J 

2.778|0.4840 

2.70310.4935! 

2.632,0.50301 

.564 0. 51241 

.5 {0.52I6J 

.439|0.5309 

38110.5400 

.326 0.5491 

273 0.5582 

2.222 0.5672 

2.1740.5761 

2.128 0.5850 

2.0830.5938 

2.04l'0.6025 

2.0 0.6112 



0130.0928 
531J0.1037 
1310.1145 
796io.l253 



0.1359 
0.1466 
0.1572 
0.1677 
0.1782 
0.1887 
0.1991 
0.2095 
0.2199 
0.2302 
0.2405 
0.2508 
0.2610 
0.2712 
0.2814 
0.2916 
0.3018! 
0.3119 
. 3220 
0.3321 
0.3422 
0.3522 
0.5623 
0.3723 
0.3823 
0.3923 



508 
259 
040 
847 
675 
519 
380 
253 
136 
029 
930 
840 
755 
676 
.603 
534 
.469 
.409 
351 
.297 
.246 
.197 
.151 
.108 
.066 
.026 
.988 
.949 
.917 
.883 
.851 
.819 
.79l|0 
.763 0.4716 
.7350.4815 
.709 0.4913 
.684 0.5012 
.659:0.5110 
.636 0.5208 



10.775 
9.643 
8.733 
7.981 
7.358 
6.821 
6.361 
5.963 
5.611 
5.299 
5.022 
4.773 
4.547 



0.0108 92.592 



4023 
4122 
4222i 
4321 
4420 
4519 
4618 



0.0147 
0.0190 
0.0237 
0.0286 
0.0337 
0.0391 
0.0447 
0.0505 
0.0566 
0.0628 
0.0692 
0.0758 
0.0825 
0.0894 
0.0965 
0.1037 
4.344 0.1111 
4.15710.1186 
3.987lo.l263 
3.83l|0.1341 
3.687i0.1420 
3.553j0.1501 
3. 429 jo. 1583 
3.313|0.1666 
3.20610.1750 
3.105J0.1836 
3.OI1I0.1922 
2.922!0.2010 
2.839J0.2099 
2.760}0.2189 
2.686 0.2281 
2.615 0.2373 
2.549 0.2466 
2.485 0.2561 
2.426|0.2656 
2.36810.2752 
2.31410.2850 
2.262I0.2948 
2.212J0.3047 
2.165i0.3148 
2.I22I0.3249 
2.0760.3351 
2.035!o.3454 



1.97f] 
1.957 
1.92C 



0.3558 
D.3663 
0.3768 



0.0157 
0.0241 
0.0327 
0270.0414 
631 0.0503 
194 0.0593 
964 0.0683 
675 0.0774 
5750.0866| 
371 0.0958 
802 0.1051 
668 0.1144 
923 0.1238 
451 0.1332 
192 0.1427 
121 0.1522 
185 0.1617 
362 0.1713 
643 0.1809 
0010.1905! 
431 0.2001! 
917 p.2098J 
457 0.2195; 
042 0.22931 
662 0,23901 
317 0.2488! 
000 0.2586 
,701 0.2684 
.446 0.2783 
.203 0.2881 
.975 0.2980 
.764 0.3079 
.5680.3178 
.384 0.3278 
.2140.3377 
.0550.3477 
.904 0.3577 
.765 0.3677 
.6330.3777 
.509 0.38781 
.392 0.3978] 
.281 0.4079 
.176 0.418o! 
.084 0.4281] 
.984 0.4382; 
.895|o. 44831 
.810:0.4585 
.730 0.4686 
.65410.4788 



63. 
41. 
30. 
24. 
19. 
16. 
14. 
12. 
11. 
10. 



64 
92 
54 
43 
,522 



0.2027 

0.2392 

0.2689 

0.29463. 

0.3173|3. 

0.33792. 

0.3568'2. 

0.3744'2 

0.3908'2 



.933 



0.321913.106 
1800. 362012. 762 
718,0. 3935|2. 541 
3940.41982.382 



.671 



0.4063 2 
0.4210|2 
741i0. 43502 



077 0.4484 
50710.4612 
007 0.4735 
57o[o.4853 2 
188]0. 49682 
83710.5079 
.527|0.5186 
.249 0.5290 
997|0.5392 
766 0.5490 
555(0. 5586 
.36110.5680 
182 0.5772 
.0190.5861 
867!o. 59491 
.725|o. 60351 
.593 0.61191 
.4710.62011 
.355 0.62821 
.247 0.63611 
.146 0.64391 
.050 0.65161 
.96l|0. 65911 
.87610.66651 
.795 1 0.67381 
.719:0.68101 
.6470.68811 
.578 0.69511 
.513 0.70191 
.451 0.70871 
.3920.71541 
.336 0.72201 
.282^.72851 
.230|0. 73491 
.18l'0. 74121 
. 1340.74751 
.0910.75371 



0.4425,2.259 
0.462712. 161 
0.4810:2.079 
0.4977:2.009 
5580. 513i:i. 948 
.46110.52751.895 
.375|o.541o|l.848 
.298j0.5537;i.806 
.2300.56571.767 
. 1680. 577l!l. 732 
.112!o. 5880 1.700 
.066!o.5984|l.67J 
.012J0.6084:i.643 
.9680.61801.618 
.930^0.6273 1.594 
.890,0. 6362 1.571 
.854*0. 6448 1.551 
.82l|0.6532|l.530 
.7920 66131.512 
.7600.66921.494 
.732J0. 6768 1.477 
.7060.68431.461 
.68l|o. 6915 1.446 
.657j0.6986'l.431 
.634 0.70551.417 
.6110.71221.404 
.591;0. 7188 1.391 
.572|0. 7252 1.378 
.553:0.73151.365 
.534:0.73771.355 
.5170.74381.344 
.5000.74971.333 
.4840.75551.323 
.4680.76121.313 
.4530.76681.304 
.4380.77231.294 
.424|0. 7777 1.285 
.411|0. 78311. 276 
.3970.78831.268 
.3850.79341.260 
.372 0.79851.252 
.3600.80351.244 
.349 0.80841.237 
.337J0.8133|l.229 
.3260.81801.222 



COMPUTATIONS IN AIR-COMPRESSION 



35 



TABLE VII.— RATIOS AND RECIPROCALS OF PRESSURE, VOLUME AND TEMPERA- 
TURE IN THE ADIABATIC COMPRESSION OF AIR.— {Continued) 



o 

1 




A 


•^1 

<p o 


B 




C 


ecip- 
ocal 




■h 


E 


.S"3 


F 


1 § 


tf ^ 




rt "^ 




tf ^ 




tf - 1 


« ** 




tf ^ 




tf - 


0.51 


1.961 


0.6199 1.6130.5306 


1.884l0.3875 


2.581 


0.4890 


2.0450.7598 


1.316 


0.8227 


1.215 


0.52 


1.923 


0.6285 1.5910.5404 


1.850 0.3982 


2.511 


0.4992 


2.003 0.7658 


1.305 


0.8274 


1.208 


0.53 


1.887 


0.6371 1.5680.5502 


1.817 0.4091 


2.443 


0.5094 


1.963 0.7718 


1.295 


0.8320 


1.201 


0.54 


1.852 


0.6456' 1.549 0.5599 


1.786 0.4200 


2.381 


0.5196 


1.924 0.7777 


1.285 


0.8365 


1.195 


0,55 


1.818 


0.6540 1.529 0.5697 


1.7550.4310 


2.320 


0.5298 


1.887 


0.7836 


1.276 


0.8409 


1.189 


0.56 


1.786 


0.6625 


1.509 0.5794 


1.725 0.4420 


2.262 


0.5401 


1.851 


0.7893 


1.266 


0.8453 


1.183 


0.57 


1.754 


0.6708 


1.491 0.5892 


1.697 0.4532 


2.206 


0.5503 


1.817 


0.7951 


1.257 


0.8497 


1.177 


0.58 


1.724 


0.6792 


1.472 0.5989 


1.669 0.4644 


2.153 


0.5606 


1.783 


0.8007 


1.248 


0.8540 


1.171 


0.59 


1.695 


0.6875 


1.4540.6086 


1.643 


0.4757 


2.102 


0.5709 


1.751 


0.8063 


1.240 


0.8582 


1.165 


0.60 


1.667 


0.6957 


1.437 


0.6183 


1.613 


0.4871 


2.051 


0.5812 


1.720 


0.8119 


1.231 


0.8624 


1.159 


0.61 


1.639 


0.7039 


1.420 0.6280 


1.592 


0.4986 


2.005 


0.5914 


1.691 


0.8174 


1.223 


0.8666 


1.153 


0.62 


1.613 


0.7121 


1.404 0.6377 


1.568 


0.5101 


1.960 


0.6018 


1.661 


0.8228 


1.215 


0.8706 


1.148 


0.63 


1.587 


0.7203 


1.388 0.6474 


1.544 


0.5218 


1.910 


0.6121 


1.633 


0.8228 


1.207 


0.8747 


1.143 


0.64 


1.563 


0.7284 


1.3720.6570 


1.522 


0.5335 


1.874 


0.6224 


1.606 


0.8335 


1.199 


0.8787 


1.138 


0.65 


1.538 


0.7364 


1.358 0.6667 


1.500 


0.5452 


1.835 


0.6327 


1.580 


0.8388 


1.192 


0.8827 


1.132 


0.66 


1.515 0.7445 


1.343 0.6763 


1.478 


0.5571 


1.795 


0.6431 


1.554 


0.8441 


1.184 


0.8866 


1.128 


0.67 


1.493:0.7524 


1.328 0.6860 


1.457 


0.5690 


1.757 


0.6534 


1.530 


0.8493 


1.177 


0.8904 


1.123 


0.68 


1.4710.7604 


1.315 


1 . 6956 


1.437 


0.5810 


1.721 


0.6638 


1.506 


0.8544 


1.170 


0.8943 


1.118 


0.69 


1.449 0.7683 


1.301 


0.7052 


1.418 


0.5931 


1.686 


0.6742 


1.483 


0.8595 


1.163 


0.8981 


1.113 


0.70 


1.429'0.7762 


1.288 


0.7148 


1.399 


0.6052 


1.652 


0.6846 


1.461 


0.8646 


1.156 


0.9018 


1.108 


0.71 


1.408 0.7841 


1.275 


0.7245 


1.380 


0.6174 


1.619 


0.6950 


1.438 


0.8696 


1.149 


0.9055 


1.104 


0.72 


1.389 0.7919 


1.263,0.7341 


1.359 


0.6297 


1.588 


0.7054 


1.417 


0.8746 


1.143 


0.9092 


1.099 


0.73 


1.3700.7997 


1.250 0.7436 


1.344 


0.6420 


1.557 


0.7158 


1.397 


0.8795 


1.137 


0.9128 


1.095 


0.74 


1.3510.8075 


1.238 


0.7532 


1.327 


0.6545 


1.527 


0.7262 


1.375 


0.8844 


1.130 


0.9165 


1.091 


0.75 


1.333 0.8152 


1.227 


0.7628 


1.311 


0.6669 


1.499 


0.7366 


1.357 


0.8893 


1.124 


0.9200 


1.087 


0.76 


1.3160.8229 


1.215 


0.7724 


1.294 


0.6795 


1.471 


0.7471 


1.338 


0.8941 


1.118 


0.9236 


1.082 


0.77 


1.299 0.8306 


1.204 


0.7819 


1.278 


0.6921 


1.444 


0.7575 


1.321 


0.8989 


1.112 


0.9271 


1.078 


0.78 


1.282-0.8382 


1.193 


0.7915 


1.263 


0.7048 


1.418 


0.7680 


1.302 


0.9036 


1.106 


0.9305 


1.074 


0.79 


1.266 0.8459 


1.182 


0.8010 


1.248 


0.7176 


1.393 


0.7785 


1.284 


0.9083 


1.100 


0.9340 


1.070 


0.80 


1.25 0.8534 


1.171 


0.8106 


1.233 


0.7304 


1.369 


0.7889 


1.267 


0.9130 


1.095 


0.9374 


1.066 


0.81 


1.235 0.8610 


1.161 


0.8201 


1.219 


0.7433 


1.345 


0.7994 


1.251 


0.9176 


0.089 


0.9408 


1.062 


0.82 


1.22 0.8685 


1.151 


0.8296 


1.205 


0.7562 


1.322 


0.8099 


1.234 


0.9222 


1.084 


0.9441 


1.059 


0.83 


1.2050.8761 


1.140 


0.8392 


1.191 


0.7692 


1.300 


0.8204 


1.219 


0.9268 


1.079 


0.9474 


1.055 


0.84 


1.19 0.8835 


1.131 


0.8487 


1.178 


0.7823 


1.278 


0.8309 


1.203 


0.9313 


1 . 073 


0.9507 


1.051 


0.85 


1.1760.8910 


1.122 0.8582 


1.165 


0.7955 


1.257 


0.8414 


1.188 


0.9358 


1.068 


0.9540 


1.048 


0.86 


1.163 


0.8984 


1.113 


0.8678 


1.152 


0.8087 


1.236 


0.8519 


1.173 


0.9403 


1.063 


0.9572 


1.044 


0.87 


1.149 


0.9058 


1.104 


0.8772 


1.139 


0.8220 


1.216 


0.8625 


1.159 


0.9448 


1 . 058 


0.9605 


1.041 


0.88 


1.136 


0.9132 


1.095 


0.8866 


1.127 


0.8353 


1.197 


0.8730 


1.145 


0.9492 


1.053 


0.9630 


1.037 


0.89 


1.124 


0.9206 


1.086 


0.8961 


1.115 


0.8487 


1.178 


0.8835 


1.131 


0.9536 


1.048 


0.9668 


1.034 


0.90 


1.111 


0.9279 


1.077 


0.9056 


1.104 


0.8621 


1.159 


0.8941 


1.118 


0.9579 


1.043 


0.9G99 


1.031 


0.91 


1.099 


0.9352 


1.069 


0.9151 


1.093 


0.8757 


1.142 


0.9047 


1.105 


0.9623 


1.039 


0.9730 


1.027 


0.92 


1.087 


0.9425 


1.061 


0.9245 


1.081 


0.8892 


1.124 


0.9152 


1.092 


0.96G0 


1.034 


0.9761 


1.024 


0.93 


1.075 


0.9498 


1.052 


0.9340 


1.070 


0.9029 


1.107 


0.9258 


1.080 


0.9708 


1.030 


0.9792 


1.021 


0.94 


1.064 


0.9570 


1.045 


0.9434 


1.059 


0.9166 


1.091 


0.9364 


1.068 


0.9751 


1 . 025 


0.9822 


1.019 


0.95 


1.053 


0.9642 


1.037 


0.9529 


1.049 


0.9303 


1.074 


0.9470 


1.056 


0.9793 


1.021 


0.9853 


1.014 


0.96 


1.0420.9714 


1.029 


0.9623 


1.039 


0.9441 


1.059i0.9576 


1.044 


0.9835 


I.OIC 


0.9882 


1.012 


0.97 


1.0310.9786 


1.021 


0.9717 


1.0290.9580 


1.043 0.9682 


1.032 


0.9877 


1.012 


0.9812 


1.009 


0.98 


1.020 0.9858 


1.014 


0.9812 


1.0190.9720 


1.0290.9788 


1.021 


0.9918 


1.008 


0.9942 


1.006 


0.99 


1.01 


0.9929 


1.007 


0.9906 


1.009 0.9860 


1.014 0.9894 


1.011 


0.9959 


1.004 


0.9971 


1.003 


1.00 


1.0 


1.0 


1.0 


1.0 


1.0 ll.O 


1.0 1.0 


1.0 


1.0 


1.0 


1.0 


1.0 




36 COMPRESSED AIR PRACTICE 

Our first and most easily accessible knowledge concerning 
any specific operation of compression is as to the pressures at 
the beginning and at the end of the compression. Thus, say 
that we are working at or near sea-level, or taking in the 
air at an absolute pressure of 1.47 lb. and delivering the air 
at 80 lb. gage, the absolute pressure then being 80+14.7 = 
94 .7 lb. ; the ratio of initial to terminal pressure will be 14 . 7 -^ 
94.7 = 0.1552. With these pressures at the opposite ends of 
the stroke the ratio will, of course, be the same whether 
compressing the air in the ordinary reciprocating compressor 
or expanding the air in doing work in an air engine or motor, 
only in one case the ratio as given would be the convenient 
multiplier in our computations and in the other case the re- 
ciprocal would apply, so that the reciprocal for every ratio is 
provided in the table as here given, for practical working 
purposes. 

The table is not entirely new, the principal elements of it 
having been computed more than 30 years ago by the late 
Richard H. Buel. The reciprocals of all the ratios have been 
added and the chart has been entirely plotted from the table, 
the smoothness of the curves in the plotting attesting the ac- 
curacy, or at least the consistency, of the figures. 

The several curves of the chart are designated by the same 
letters as the columns of the table to which they correspond, 
adiabatic compression, or expansion, being assumed throughout. 
The several series of ratios, whether in the curves of the chart, 
or in the columns of the table are as follows: 

A = Ratios of initial and terminal volumes of air for given 
ratios of absolute pressures. 

B = Ratios of initial and terminal volumes of saturated steam 
for given ratios of absolute pressures. 

C = Ratios of initial and terminal absolute pressures of air for 
given ratios of volumes. 

D = Ratios of initial and terminal absolute pressures of satu- 
rated steam for given ratios of volumes. 

£' = Ratios of initial and terminal absolute temperatures of 
air for given ratios of volumes. 

F = Ratios of initial and terminal absolute temperatures of 
air for given ratios of absolute pressures. 

The formulas follow by which the ratios in the several columns 

were computed, p being the ratio in the first colmnn of the table. 



37 



1.00 



0.90 



0.80 



0.70 



b 0.60 



ing 

be 

is 

ery 



^. 0,50 
a> 



S 

V 0.40 



red 
ise- 
cal 
of 
ngs 
art 
I F 
red 



0.30 



0^0 



0.10 



ur- 
vel 
the 
•es- 
3ry 
vill 
me 
iof 
ing 
Dm 
lau 
his 
,tle 



1 



1.00 
0.90 


^^^H 

^^^^^H 


«■ 




-±--3^- 


-^-'f^ 


w 


^^^^sffl^^ffiffl 


^f^^^^^ffi 




:i;S:~S 




::;^--±-i--- 




^^^^Mffl 




- = ::^S 


ll-l 


mi 




+:=:^==:=ii^i=^==^===^^ 


: -: ::: ::-,D§g3^---- 


W- 


^^: = #: 


?S::: 




::i:S|:i|^:::::::::: 


+ :::::: + ::::;:::^^^ 






r----""""''^^i-ffi^^| 


^^:^:^ 




j:::::±:^- 






E::::::::::|--:::::::;;:::::;,fl 


^,^-|:::E: 






0.70 


|::::=i::;:::i|::i: 




"-S^S- 




:; " 


1 


|--^|:|#:::|:| 


^^rwJ Ip^ffif^tlttti 




g::: 


:::::S±::: 


L 


= = =^^|^=g:;i:fc::;:^; 


^^^H 




^^^^ 


= -T--4--i-:>ykE::::::: 






^=^^^^^|:::|l 


; 


Si^ff^f^#fflH 


: ---fWC " ■ ^^4f fl^ - " " '^ 


j::-:E 




mm 


:::i:::::;:i?i;::::::::::: 


^:::|;?^|;:-^;= 


1- 


|:::::::;^!::::::::::::::|i 


|:|||i:±^ffi:- 


-m 




:::^g:-|:^ + 










i^^K 


y;;:;::;;;;;;;::« 




■^Wf 


|::e::::=:::I;i|:=::::^:: 


:-#::i::: 


j- 1 1 1 1 1 1 1 1 1 1 1 1 




::i^:#l?=:;;::^^:::;:::;: 


±^_3:_!E^S + _]r&j5^ 


._ — 5. .J 


::::::::::::::::::- 


0.20 


































































0.10 


w^m 


pj^jf^kMP n 




;:i;'^i;;;::;;;;;:; 




-tr^ i;„„d: 




:::i::^ 


; : 







0.0 0.10 0.20 0.30 0.40 0.60 0.60 0.70 

1.00 0.90 0.80 0.70 0.60 0.60 0.40 0.30 

Given Ratios of Pressure, Volume or Temperature. 

Fia. 6.— Ratios of Initial and Final Pressures, Volumes and Temperatures. 



36 



Oi 
any 
the 1 
that 
air a 
at 8' 
94.7 
94.7 
the 
comj 
or e:s 
only 
mult 
cipro 
prov: 
purp' 

Tl: 
havh 
Rich 
adde 
the ^ 
curac 

Tt 
lette] 
a dial 
The 
or in 

A- 
ratio; 

B- 
for g 

C-- 
giver 

D- 
rated 

E- 
air f c 

F = 
air fc 

Tt 



were 



COMPUTATIONS IN AIR-COMPRESSION 37 

^ &" 



^ ©" 

« ©'■*" ■ 

" ©'» 

- ar 

^ (ir 

To anyone appreciating and capable of intelligently using 
the table, any explanation of methods of procedure would be 
superfluous. The table, although somewhat voluminous is 
essentially simple, as is also the chart which embodies it in every 
particular. 

In the chart the given or known ratios, from which the desired 
results are to be obtained, are noted horizontally on the base- 
line, and the ratio desired is then indicated by the vertical 
distance from the base-line at the point noted to the curve of 
ratios required in the particular case, the scale for the readings 
being at the side of the chart The curves A, B and E start 
from the lower left-hand corner, and the curves C, D and F 
from the right-hand corner, the base-line being double figured 
for facility of reading in either direction. 

As illustrating the use of the chart, say that we have an air- 
compressor working at an altitude of 2000 ft. above sea-level 
compressing 900 cu. ft. of air per minute and delivering it into the 
receiver at 75-lb. gage, pressure. The normal atmospheric pres- 
sure at 2000 ft. being, say, 13.7 lb., and the absolute delivery 
pressure then being 75 + 13.7 = 88.7 lb., the pressure ratio will 
be 13 . 7 -^ 88 . 7 = . 1544 To ascertain now the delivery volume 
of the air compressed, we take this ratio, the first two figures of 
it as they stand and the other two figures by approximating 
between the lines, and locate it on the base-line reading from 
the left-hand corner. Taking 15 spaces and a little less than 
half of the next space we note the vertical distance from this 
point up to curve A, and this we find to be 0.20 with a little 



38 COMPRESSED AIR PRACTICE 

more than half of the next vertical space, say, 0.26 5/8, or, deci- 
mally, 0.2662, and this being the ratio of the terminal to the 
initial volume, the actual terminal volume per minute will be 
900X0.2662 = 239.58 cu. ft. at 75 lb. gage. 

The volume thus ascertained is, of course, the volume actually 
delivered and at the moment of delivery, assuming that the 
compression was perfectly adiabatic throughout, or that the 
body of air, while being compressed, neither lost nor gained 
any heat by conduction, radiation or otherwise; and, as a matter 
of fact, this is nearly the actual state of affairs in ordinary com- 
pression. If, during compression, the air could be maintanied 
at constant temperature the ratios of volumes would then be 
practically the same as the ratios of pressures, and the ''curve" 
for this would be a straight diagonal from the lower left-hand 
to the upper right-hand corner. 

To ascertain now the ratio of temperatures as the result of 
our compression the operation is similar, except that in this case 
we use curve F instead of curve A . Taking our ratio of pressure 
0.1544, with which we started, but this time reading from the 
right-hand corner we find the vertical reading to curve F to be 
0.58, or so little above that the excess is negligible. We nete 
in the table that the riciprocal of 0.58 is 1.724, and this it will 
be convenient to use here. If the initial temperature of the air 
by the thermometer was 60° then the absolute initial tempera- 
ture would be 60+461 = 521, and the terminal temperature 
would be 521X1.724 = 898 absolute, or 898-461=473° by the 
thermometer. 

If we take our ratio of volumes . 2662, as previously ascer- 
tained and, reading from the left-hand corner, note the vertical 
reading to curve E, the ratio of temperatures will be found to 
be the same as that obtained above from curve F. 

If we take any horizontal line of the chart, or any horizontal 
line arbitrarily placed upon the chart, and note the points where 
curves E and F cut it then the distance from curve E to the left- 
hand vertical boundary will be the ratio of volumes for that 
temperature ratio, and the distance from curve F to the right- 
hand boundary will be the ratio of pressures, so that all particulars 
embodied in curves A and C are really obtainable from curves E 
andF. 

Mean Effective Pressures in Compression or Expansion of 
Air. — The following table, VIII, and diagram, Fig. 7, in connection 



COMPUTATIONS IN AIR-COMPRESSION 39 

with the preceding, offer special facilities for power computations 
in connection with the compression or the working expansion 
of air. The former table and diagram dealt with the interde- 
pendent ratios of pressm-e, volume and temperature, either of 
which being given the others were readily derivable therefrom. 

In the table and diagram here presented the given ratio is that 
of the volumes at the beginning and the end of the stroke. From 
that is readily determined the mean effective pressure ratio, 
either during the entire stroke or during compression or expan- 
sion only. On the diagram, the given ratio of volumes is read 
upon the base-line and the required ratio of the mean effective 
pressures is then indicated by the vertical height from the base- 
line at the point thus indicated up to the curve representing 
the required data. It is assumed that no further explanation is 
required. 

In computations relating to the power developed in the working 
expansion of air in a reciprocating engine or motor, the ratio of 
volumes is the particular most readily available, as that is deter- 
mined by the point of cut-off, but in air compression we first 
know the ratio of pressures, and the ratio of volumes is derivable 
from this by the assistance of the previous table or diagram, so 
that both tables or diagrams are necessary to constitute a com- 
plete equipment, and they should be kept together. Of course 
only absolute pressures are dealt with, and when mean effective 
pressures are obtained, these also are absolute, and the actual 
working mean effective is obtained by deducting the pressure of 
the atmosphere. In compound compression or expansion, for 
the high-pressure cylinder there must be subtracted the total 
absolute back pressure, which of course must be much greater 
than that of the atmosphere alone. 

The several columns of the table designated by capital letters, 
and the corresponding lines of the diagram, represent the follow- 
ing ratios of mean to initial, or terminal, total pressures, the given 
ratio in each case being the ratio of volumes. 

FOR ENTIRE STROKE 

H, Perfect gas, temperature constant. 

J, Air expanding (or being compressed) without loss or gain of 
heat. 
K, Saturated steam. 



40 



COMPRESSED AIR PRACTICE 



TABLI5 VIII. 



-RATIOS AND RECIPROCALS OF MEAN EFFECTIVE PRESSURES 
FOR GIVEN RATIOS OF VOLUMES 



s° 


d 




d 




d 




d 




d 




_d 


d 


.tt 


"§ 


H 


'i 


J 


1 


K 


'i 


L 




M 


Qi 


N 'i 


a2 






tf 




tf 




a 




« 




Pi 


tf 


0.01 


100.0 


0.0561 


17.825 


0.0308 


32.467 


0.0500 


20.000 0.0465 


21.505 


0.0210 


47.619 


0.0404 24.752 


0.02 


50.0 


0.0982 


10.018 


0.0591 


16.920 


0.0894 


11.185 


0,0798 


12.531 


0.0399 


25,313 


0.0708 


14.124 


0.03 


33.33 


0.1352 


7.395 


0.0860 


11.628 


0.1245 


8.032 


0,1085 


9.217 


0.0577 


17.330 


0.0974 


10.266 


0.04 


25.00 


0.1688 


5.924 


0.1171 


8.952 


0.1566 


6.385 


0.1341 


7.457 


0.0747 


13.387 


0.1215 


8.230 


0.05 


20.00 


0.1998 


5.005 


0.1365 


7.326 


0.1866 


5.359 


0.1577 


6.334 


0.0910 


10.989 


0.1438 


6.954 


0.06 


16.667 


0.2288 


4.371 


0.1604 


6.234 


0,2148 


4.655 


0.1796 


5.567 


0.1068 


9.363 


0.1647 


6.071 


0.07 


14.286 


0.2562 


3.903 


0.1836 


5.446 


0.2415 


4.140 


0.2002 


4.995 


0.1222 


8.183 0.1844 


5.423 


0.08 


12.50 


0.2821 


3.544 


0.2061 


4.852 


0.2669 


3,746 


0.2196 


4.553 


0.1371 


7.294 0.2032 


4.921 


0-.09 


11.111 


0.3067 


3.260 


0.2280 


4.386 


0.2912 


3.434 


0.2382 


4.198 


0.1517 


6.592 


0,2211 


4.522 


0.1,0 


10.00 


0.3303 


3.027 


0.2493 


4.011 


0.3145 


3.179 


0.2558 


3.909 


0.1659 


6.027 


0.2383 


4.280 


0.11 


9.09 


0.3528 


2.834 


0.2701 


3.702 


0.3368 


2.969 


0.2728 


3.665 


0.1798 


5.561 


0.2548 


3.924 


0.12 


8.333 


0.3744 


2.679 


0.2903 


3.444 


0.3583 


2.791 


0.2891 


3.459 


0.1935 


5.167 


0.2708 


3.692 


0.13 


7.692 


0.3952 


2.533 


0.3100 


3.258 


0.3790 


2.638 


0.3049 


3.279 


0.2069 


4.833 


0.2862 


3.494 


0.14 


7.143 


0.4153 


2.407 


0.3293 


3.036 


0.3990 


2.506 


0.3201 


3.124 


0.2201 


4.543 


0.3012 


'3.320 


0.15 


6.667 


0.4346 


2.301 


0.3481 


2.872 


0.4183 


2.390 


0.3348 


2.986 


0.2331 


4.290 


0.3157 


3.167 


0.16 


6.25 


0.4532 


2.206 


0.3665 


2.728 


0.4370 


2.288 


0,3491 


2.864 


0.2458 


4.068 


0.3298 


3.032 


0.17 


5.882 


0.4712 


2.122 


0.3845 


2.601 


0.4552 


2.197 


0,3629 


2.755 


0.2584 


3.870 


0.3436 


2.910 


0.18 


5.556 


0.4887 


2.046 


0.4020 


2.487 


0.4727 


2.115 


0.3764 


2.656 


0.2708 


3.692 


0.3570 


2.801 


0.19 


5.263 


0.5055 


1.978 


0.4192 


2.385 


0.4897 


2.042 


0.3896 


2.567 


0.2830 


3,533 


0.3700 


2.702 


0.20 


5.00 


0.5219 


1.904 


0.4360 


2.293 


0.5062 


1.975 


0.4024 


2.485 


0.2950 


3.390 


0.3828 


2.612 


0.21 


4.762 


0.5377 


1.859 


0.4524 


2.210 


0.5223 


1.914 


0.4149 


2.410 


0.3069 


3.258 


0.3953 


2.529 


0.22 


4.545 


0.5531 


1.808 


0.4685 


2.134 


0.5378 


1.859 


0.4271 


2.341 


0.3186 


3.138 


0.4075 


2.454 


0.23 


4.348 


0.5680 


1.760 


0.4842 


2.065 


0.5530 


.1.808 


0.4390 


2.277 


0.3302 


3.028 


0.4194 


2.384 


0.24 


4.167 


0.5825 


1.716 


0.4996 


2.001 


0.5677 


1. 761 10. 4507 


2.209 


0.3416 


2.927 


0.4312 


2.319 


0.25 


4.P0 


0.5966 


1.676 


0.5147 


1.942 


0.5820 


1.718 


0.4621 


2.164 


0.3529 


2.833 


0.4426 


2.259 


0.26 


3.846 


0.6102 


1.638 


0.5295 


1.888 


0.5959 


1.678 


0.4733 


2.112 


0.3641 


2.7"46 


0.4539 


2.20.'$ 


0.27 


3.704 


0.6235 


1.604 


0.5439 


1.838 


0.6094 


1.641 


0.4843 


,2.066 


0.3752 


2.665 


0.4650 


2.150 


0.28 


3.571 


0.6364 


1.571 


0.5580 


1.792 


0.6226 


1.606 


0.4950 


\2.020 


0.3861 


2.590 


0.4759 


2.101 


0.29 


3.448 


0.6490 


1.541 


0.5718 


1.748 


0.6355 


1.573 


0.5056 


1.977 


0.3970 


2.519 


0.4866 


2.055 


0.30 


3.333 


0.6612 


1.512 


0.5854 


1.708 


0.6479 


1.543 


0.5160 


1.938 


0.4077 


2.453 


0.4971 


2.011 


0.31 


3.226 


0.6731 


1.485 


0.5986 


1.670 


0.6601 


1.515 


0.5262 


1.900 


0.4183 


2.391 


0.5074 


1.970 


0.32 


3.125 


0.6846 


1.461 


0.6116 


1.635 


0.6719 


1.488 


0.5362 


1.865 


0.4288 


2.332 


0.5176 


1.932 


0.33 


3.03 


0.6959 


1.437 


0.6243 


1.602 


0,6835 


1.463 


0.5461 


,1.831 


0.4393 


2.276 


5276 


1.895 


0.34 


2.941 


0.7068 


1.414 


0.6367 


1.570 


0.6947 


1.439 


0.5558 


1.799 


0.4496 


2.224 


0.5374 


1.861 


0.35 


2.857 


0.7174 


1.394 


0.6489 


1.541 


0.7056 


1.417 


0.5653 


1.768 


0.4598 


2.177 


0.5471 


1.827 


0.36 


2.778 


0.7278 


1.374 


0.6608 


1.513 


0.7163 


1.396 


0.5747 


1.757 


0.4700 


2.127 


0.5567 


1.796 


0.37 


2.703 


0.7379 


1.355 


0.6724 


1.487 


0.7267 


1.376 


0.5839 


1.712 


0.4800 


2.083 


0.5662 


1.766 


0.38 


2.632|0.7477 


1.337 


0.6838 


1.462 


0.7368 


1.357 


0.5930 


1.686 


0.4900 


2.040 


0.5755 


1.737 


0.39 


2.564 


0.7572 


1.321 


0.6949 


1.439 


0.7466 


1.339 


0.6020 


1.661 


0.4999 


2.000 


0.5846 


1.710 


0.40 


2.50 


0.7665 


1.304 


0.7058 


1.416 


0.7562 


1.322 


0.6109 


1.636 


0.5097 


1.961 


0.5937 


1.684 


0.41 


2.439 


0.7756 


1.289 


0.7165 


1.395 


0.7656 


1.306 


0.6196 


1.613 


0.5194 


1.925 


0.6026 


1.659 


0.42 


2.381 


0.7844 


1.274 


0.7269 


1.375 


0.7747 


1.291 


0.6282 


1.592 


0.5291 


1.890 


0.6115 


1.633 


0.43 


2.326 


0.7929 


1.261 


0.7370 


1.356 


0.7835 


1.276 


0.6367 


1.570 


0.5386 


1.856 


0.6202 


r.612 


0.44 


2.273 


0.8012 


1.248 


0.7470 


1.338 


0.7921 


1.262 


0.6451 


1.550 


0:5481 


1.824 


0.6288 


1.590 


0.45 


2.222 


0.8093 


1.235 


0.7567 


1.321 


0.8005 


1.249 


0.6533 


1.530 


0.5576 


1.793 


.06373 


1.569 


0.46 


2.174 


0.9172 


1.223 


0.7662 


1.305 


0.8087 


1.236 


0.6615 


1.510 


0.5670 


1.763 


0.6457 


1.548 


0.47 


2.128 


0.8249 


1.212 


0.7754 


1.289 


0.8166 


1.224 


0.6696 


1.493 


0.5762 


1.735 0,6540 


1.529 


0.48 


2.083 


0.8323 


1.201 


0.7845 


1.274 


0.8244 


1.213 


0.6775 


1.476 


0,5855 


1.708 0,6622 


1.510 


0.49 


2.041 


0.8395 


1.191 


0.7933 


1.265 


0.8319 


1.202 


0.6854 


1.459 


0,5947 


1.6810.6703 


1.492 


0.50 


2.00 


0.8466 


1.181 


0.8019 


1.247 


0.8392 


1.1910.6932 


1.442 


0.6038 


1.656 0.6784 


1.474 



1.00 



0.90 



S 0.80 



I 0.70 



•3 0.60 



.b 0.50 



79 
65 
52 
38 
26 

13 

01 
89 
78 
66 

56 
45 
35. 
24 
15 

05 
96 



•So.- 



0.30 



I 0.20 



61 
52 
44 
36 
28 

20 
13 
05 
^8 
'91 



S 0.10 
S 

I 



184 
•77 
•71 
(63 
158 

•51 
»44 
•39 
•33 
•277 



•219 
• 162 
(106 
K)53 
)0 



nw 


]J|ffl14^ 




IMM 


0.80 
0.70 
0.60 


±":::±:::::::;:::;|5;;::::::::: 

Ti^ijj r 1 

::::::::::;:::::::jfii^?:^3: 






0.50 
0.40 
0.30 






WW 


0.20 




---? ii--- 


#t+#mwH=H-ii 


0.10 


■Mi 


1 r^^T^ — ^ — 

:::I:::::::::::::I::::::::::i:±:::::::::: 


Wrin 



1.40 



0.70 



5 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.36 0.30 0.25 0.20 0.15 0.10 0.05 0.0 
Eatioa of Volumes-Upper Line of Figures for H, J, L-, M. Lower Line for K and N. 

Fie. 7. — Ratios of Mean Efifective Pressures in Compression or Expansion to Given Ratios of Volumes. 

^ i- J- (facing Page iO.) 



40 


' 


TAE 


a 


0.01 10( 


0.02 


5( 


03 


3; 


0.04 


21 


0.05 


2( 


0.06 


1( 


0.07 


1^ 


0.08 
0-.09 


11 


0.10 


i( 


0.11 


< 


0.12 


i 


0.13 


', 


0.14 


' 


0.15 


{ 


0.16 


i 


0.17 


I 


O.IS 


,' 


0.19 


,' 


0.20 


.' 


0.21 


< 


0.22 




0.23 




0.24 
0.25 




0.26 


. 


0.27 




0.28 




0.29 


, 


0.30 


. 


0.31 


; 


0.32 


; 


0.33 


J 


0.34 


i 


0.35 




0.36 


• 


0.37 


, 


0.38 




0.39 




0.40 


1 


0.41 


; 


0.42 


• 


0.43 


\ 


0.44 


\ 


0.45 


'. 


0.46 
0.47 


•J.. 


0.48 


\ 


0.49 




0.50 









COMPUTATIONS IN AIR-COMPRESSION 



41 



TABLE VIII. 



-RATIOS AND RECIPROCALS OF MEAN EFFECTIVE PRESSURES 
FOR GIVEN RATIOS OF VOLUMES— Continued 



g.2 


o. 




a 




'd 




.a 




d 




.s- 




d 


.t^ 


'i 


H 


"i 


J 


'i 


K 




L 


1 


M 


e 


N 




o^ 


« 




« 




tf 




•tf 




. tf 




« 




« 


0.51 


1.961 


0.8534 1.171 


0.8103 


1.234 


0.8463 1.181 


0.7008 


1.426 


0.6128 


1.631 


0.6863! 1.457 


0.52 


1.923 


0.86001.163 


0.8185 


1.222 


0.8532 1.172 


0.7084 


1.410 


0.6218 


1.608 


0.6941 11.441 


0.53 


1.887 


0.86651.154 


0.8264 


1.210 


0.8599,1.162 


0.7159 


1.396 


0.6307 


1.585 


0.7019 


1.424 


0.54 


1.852 


0.872711.147 


0.8342 1.191 


0.8664 


1.154 


0.7234 


1.382 


0.6396 


1.563'0.7096 


1.409 


0.65 


1.818 


0.87881.137 


0'. 8418 1.188 


0.8727 


1.145 


0.7307 


1.368 


0.6484 


1.542 0.7172 


1.394 


0.56 


1.786 


0.884711.130 


0.8492 1.177 


0.8789 


1.137 


0.7380 


1.355 


0.6572 


1.5210.7247 


1.379 


0.57 


1.754 


0.8904 1.123 


0.8563 1.167 


0.8848 


1.130 


0.'7451 


1.342 


0.6659 


1.5010.7322 


1.365 


0.58 


1.724 


0.8959 


1.116 


0.8633,1.158 


0.8906 


1 . 122 


0.7522 


1.329 


0.6745 


1.482 0.7396 


1.352 


0.59 


1.695 0.9013 


1.109 


0.8701 


1.149 


0.8962 


1.115 


0.7593 


1.317 


0.6831 


1.463 0.7469 


1.338 


0.60 


1.667 0.9065 


1.103 


0.8767 


1.140 


0.9017 


1.109 


0:7662 


1.305 


0.6917 


1.445 0.7541 


1.326 


0.61 


1.6390.9115 


1.097 


0.8831 


1.130 


0.^069 


1, 102 


0:7731 


1.293 


0.7002 


1.428'0.7613 


1.313 


0.62 


1.613 


0.9164 


1.091 


0.8893 


1.124 


0.9120 


1.096 


0.7800 


1.282 


0.7086 


1.411|0.7684 


1.301 


0.63 


1.587 


0.9211 


1.085 


0;8953 


1.116 


0.9169 


1.090 


0.7867 


1.271 


0.7170 


1.394 


0.77.55 


1.289 


0.64 


1.563 0.9256 


1.080 


0.9011 


1.109 


0.9217 


1.085 


0.7934 


1.259 


0.7254 


1.378 


0.7824 


1.278 


0.65 


1.538 0.9300 


1.075 


0.9068 


1.102 


0.9263 


1.079 


0.8000 


1.250 


0.7337 


1.362 


0.7894 


1.266 


0.66 


1.515 


0.9342 


1.070 


0.9123 


1.096 


0.9307 


1.074 


0.8066 


1.239 


0.7419 


1.347 


0.7962 


1.256 


0.67 


1.493 


0.9383 


1.085 


0.9176 


1.089 


0.9350 


1.089 


0.8131 


1.229 


0.7501 


1.333 


0.8030 


1.245 


0.68 


1.471 


0.9423 


1.081 


0.9227 


1.083 


0.9391 


1.084 


0.8196 


1.220 


0.7583 


1.320 


0.8097 


1.235 


0.69 


1.449 


0.9460 


1.057 


0.9276 


1.078 


0.9431 


1.080 


0.8259 


1.210 


0.7664 


1.304 


0.8164 


1.224 


0.70 


1.429 


0.9497 


1.053 


0.9324 


1.072 


0.9469 


1.056 


0.8322 


1.201 


0.7745 


1.291 


0.8230 


1.215 


0.71 


1.408 


0.9532 


1.049 


0.9369 


1.087 


0.9506 


1.052 


0.8385 


1.192 


0.7826 


1.277 


0.8296 


1.205 


0.72 


1.389 


0.9565 


1.045 


0.9414 


1.082 


0.9541 


1.048 


0.8447 


1.183 


0.7906 


1.26l|0.8361 


1.196 


0.73 


1.370 


0.9597 


1.042 


0.9456 


1.057 


0.9575 


1.044 


0.8509 


1.175 


0.7885 


1.252|0.8426 


1.186 


0.74 


1.357 


0.9628 


1.038 


0.9497 


1.053 


0.9607 


1.040 


0.8570 


1.166 


0.8064 


1.240 0.8490 


1.177 


0.75 


1.333 


0.9658 


1.035 


0.9536 


1.048 


0.9338 


1.037. 


0.8631 


1.158 


0.8143 


1.228 


0:8553 


1.169 


0.76 


1.316 


0.9686 


1.032 


0.9573 


1.044 


0,9868 


1.034 


0.8691 


1.150 


0.8221 


1.216 


0.8616 


1.161 


0.77 


1.299 


0.9713 


1.029 


0.9609 


1.040 


0.9896 


1.0.30 


0.8750 


1.142 


0.8300 


1.204 


0.8679 


1.152 


0.78 


1.282 


0.9738 


1.026 


0.9643 


1.036 


0.9723 


1.028 


0.8809 


1.135 


0.8377 


1.193 


0.8741 


1.144 


0.79 


1.266 


0.9762 


1.024 


0.9675 


1.033 


0.9749 


1.025 


0.8868 


1.127 


0.8455 


1.182 


0.8803 


1.136 


0.80 


1.25 


0.9785 


1.022 


0.9706 


1.030 


0.9773 


1.023 


0.8926 


1.120 


0.8532 


1.172 


0.8864 


1.128 


0.81 


1.235 


0.9807 


1.020 


0.9736 


1.027 


0.9796 


1.021. 


0.8983 


1.113 


0.8608 


1.161 


0.8925 


1.120 


0.82 


i.22 


0.9827 


1.017 


0.9763 


1.024 


0.9817 


1.018 


0.9041 


1.106 


0.8684 


1.151 


0.8985 


1.113 


^0.83 


1.205 


0.9847 


1.015 


0.9789 


1.021 


0.9838 


1.016 


0.9097 


1.099 


0.8760 


1.1410.9045 


1.105 


0.84 


1.19 


0.9865 


1.013 


0.9814 


1.019 


0.9857 


1.014 


0.9154 


1.092 


0.8836 


1.129 


0.9104 


1.098 


0.85 


1.176 


0.9881 


1.012 


0.9837 


1.016 


0.9874 


1.012 


0.9209 


1.085 


0.8911 


1.122 


0.9162 


1.091 


0.86 


1.163 


0.9897 


1.0104 


0.9858 


1.014 


0.9891 


1.010 


0.9265 


1 079 


0.8986 


1.112 


0.9221 


1.084 


0.87 


1.149 


0.9912 


1.0089 


0.9878 


1.012 


0.9906 


1.009 


0.9320 


1.072 


0.9060 


1.103 


0.9279 


1.077 


•0.88 


1.136 


0.9925 


1.0075 


0.9896 


1.010 


0.9920 


1.008 


0.9375 


1.066 


0.9134 


1.894 


0.9337 


1.071 


0.89 


1.124 


0.9937 


1,0061 


0.9913 


1.0087 


0.9933 


1.0067 


0.9429 


1.060 


0.9208 


1.086 0.9394 


1.063 


0.90 


1.111 


0.9948 


1.0052 


0.9928 


1.0072 


0.9945 


1.0055 


0.9482 


1.054 


0.9282 


1.077 0.9451 


1.058 


0.91 


1.099 


0.9958 


1.0042 


0.9942 


1.0058 


0.9956 


1.0044 


0.9536 


1.048 


0.9365 


1.069 0.9508 


1.051 


0.92 


1.087 


0.9967 


1.0033 


0.9954 


1.0046 


0.^965 


1.0035 


0.9589 


1.042 


0.9428 


1 060!0.9564 


1.044 


0.93 


1.075 


0.9975 


1.0025 


0.9965 


1.0035 


0.9973 


1.0027 


0.9642 


1.037 


0.9500 


1.05210.9620 


1.039 


0.94 


1.064 


0.9982 


1.0018 


0.9974 


1.0026 0.9981 


1.0019 


0.9694 


1.031 


0.9572 


1.044 0.9676 


1.033 


0.95 


1.053 


0.9987 


1.0013 


0.9982 


1.0018 0.9987 


1.0013 


0.9746 


1.026 


0.9645 


1.036 


0.9730 


1.0277 


0.96 


1.042 


0.9992 


1.0008 


0.9989 


1.00110.9991 


i:D009 


0.9797 


1.0207 


9716 


1.029 


0.9785 


1.0219 


0.97 


1.031 


0.9996 


1.0004 


0.9994 


1.0006 0.9995 


1.0005 


0.9849 


1.0155 


9788 


1.0210.9840 


1.0162 


0.98 


1.02 


0.9998 


1.0002 


0.9997 


1.0003 0.9998 


1.0002 


0.9899 


1.0102 


0.9859 


1.014 0.9895 


1.0106 


0.99 


1.01 


0.9999 


1.0001 


0.9999 


1.00010.9999 


1.0001 


0.9950 


1.005 


0.9929 


1.007 0.9947 


1.0053 


1.00 


1.00 


1.00 


1.00. 


1.00 


1.00 1.00 


1.00 


1.00 


1.00 


1.00 


1.00 1.00 


1.00 



42 COMPRESSED AIR PRACTICE 

DURING EXPANSION OR COMPRESSION ONLY 

L, Perfect gas, temperature constant. 

M, Air expanding (or being compressed) without loss or gain 
of heat. 

N, Saturated steam. 

The following are the formulas by which the computations 

were made, p- being the given ratio : 

l-\-hyp. log. R 
^ R 

/ 1 \ 0.408 

J, 3.451-2.451+ y 

R 



hyp, log. R 

^ "B-r~ 

R-1 

R-l 

Habits of computation vary with the individual, some paying 
more attention to minute points than others, and some doing 
mentally what would compel a liberal use of pencil and paper by 
others. In any case the detailed descriptions of arithmetical 
operations are apt to make them appear more complicated than 
they really are. 

We will take here a very simple case of single-stage compres- 
sion. Say that we have a straight-line compressor whose air 
cylinder is 24-in. diameter and 36-in. stroke, piston rod 4 in., 
running at 72 r.p.m. and compressing the air to 70 lb., gage; 
what will be the theoretic horse-power required for adiabatic 
compression? The area of a 24-in. circle is 452.39 sq. in. and 
of a 4-in. is 12.56. Then (452.39+452.39-12.56) -^2 = 446 sq. 
in., as the mean piston area for both strokes. 

As the compression is to 70 lb., gage, the absolute pressure is 



COMPUTATIONS IN AIR-COMPRESSION 43 

70+14.7 = 84.7, and the pressure ratio is 14.7-^84.7 = 0.1736. 
Referring to Table VII, to get precisely the ratio of volumes, 
using column A, we find that for 0.17, or 0.1700, it is 0.2841, 
and for 0.1800 it is 0.2959, the difference being 0.2959-0.2841 = 
0.0118, then 0.0118X0.36 = 0.004248, and 0.2841+0.0042 = 
0.2883 as the precise ratio of volumes. 

Coming now to the table before us, using column /, we find that 
for 0.2800 volume ratio, the mean effective pressure ratio is 
0.5580 and for 0.2900 it is 0.5718, the difference being 0.0138, 
then 0.0138X0.83 = 0.0114 and 0.5580+0.0114 = 0.5694 as the 
precise mean effective pressure ratio. 

It is usually not necessary, or, indeed worth while, to take all 
this pains to get the last two figures of the ratio, and in using the 
diagrams alone the operation would be much simpler. The 
pressure ratio, 0.1736, we would fix in our minds as 0.17 1/3 + , 
and finding this point on the base-line of the previous diagram. 
Fig. 6, we would read the vertical distance up to curve A as 
0.28 7/8 — . Then turning to the diagram now before us, with 
0.28 7/8 — on the base-line, we would read the vertical distance 
to curve J as 0.57, which would be sufficiently close to the 0.5694 
obtained from the table. 

The absolute terminal pressure of compression being 84.7, the 
absolute mean effective pressure is 84.7X0.5694 = 48.23, and the 
working mean effective is 48.23 — 14.7 = 33.5. 

The theoretic horse-power then is 446, mean piston area, X 
33.5, mean effective resistance, X 6 ft., per double stroke, X 72 
r.p.m. -^ 33,000 ft.-lb. = 196 h.p., to which is to be added 
for friction whatever percentage information may warrant or 
judgment suggest. 

A HANDY TABLE FOR SIMPLE COMPRESSION COMPUTATIONS 

Table IX was produced by the aid of those preceding it. 
The writer has used it in his own practice for many years, and 
its value for general use has been attested by its frequent re- 
production in pocket books, air machinery builders' catalogues, 
etc. It is serviceable in many ways in general computations 
relating to the compression of air and requires little explanation. 
Throughout the table the air is assumed to be compressed from 
the normal pressure of 1 atmosphere, 14.7 lb. absolute, and from 
an initial temperature of 60° F. The first three columns of the 



44 



COMPRESSED AIR PRACTICE 



TABLE IX.— VOLUMES, MEAN PRESSURES, TEMPERATURES, ETC., IN THE 
OPERATION OF AIR COMPRESSION FROM I ATMOSPHERE AND 60° FAHR. 



1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


" 2 

a 


ft 


! 


I B 

t ^ ^ 
S 1 2 
3 « 
^ ft 




s ^ 
3 

> 


aos3 


II, 

ID ^ 

ft ^ 03 

ill 


S .1 « 1 

1 g " i 
s a. 53 ft 

go a> 
ft " . 2 


l.sli 


ft 

a .a 

-.it 
Us 


03 
3 

ft 
<D 







14.7 


1.0 


1.0 


1.0 


0.0 


0.0 


0.0 


0.0 


60.0 





1 


15.7 


1.068 


0.9363 


0.95 


0.96 


0.975 


0.43 


0.44 


71.0 


1 


2 


16.7 


1.136 


0.8803 


0.91 


1.87 


1.91 


0.95 


0.96 


80.4 


2 


3 


17.7 


1.204 


0.8305 


0.876 


2.72 


2.8 


1.4 


1.41 


88.9 


3 


4 


18.7 


1.272 


0.7861 


0.84 


3.53 


3.67 


1.84 


1.86 


98.0 


4 


5 


19.7 


1.34 


0.7462 


0.81 


4.3 


4.5 


2.22 


2.26 


106.0 


5 


10 


24.7 


1.68 


0.5952 


0.69 


7.62 


8.27 


4.14 


4.26 


145.0 


10 


15 


29.7 


2.02 


0.495 


0.606 


10.33 


11.51 


5.77 


5.99 


178.0 


15 


20 


34.7 


2.36 


0.4237 


0.543 


12.62 


14.4 


7.2 


7.58 


207.0 


20 


25 


39.7 


2.7 


0.3703 


0.494 


14.59 


17.01 


8.49 


9.05 


234.0 


25 


30 


44.7 


3.04 


0.3280 


0.4638 


16.34 


19.4 


9.66 


10.39 


255.0 


30 


35 


49.7 


3.381 


0.2957 


0.42 


17.92 


21.6 


10.72 


11.59 


281.0 


35 


40 


54.7 


3.721 


0.2687 


0.393 


19.32 


23.66 


11.7 


12.8 


302.0 


40 


45 


59.7 


4.061 


0.2462 


0.37 


20.52 


25.59 


12.62 


13.95 


321.0 


45 


50 


64.7 


4.401 


0.2272 


0.35 


21.79 


27.39 


13.48 


15.05 


339.0 


50 


55 


69.7 


4.741 


0.2109 


0.331 


22.77 


29.11 


14.3 


15.98 


357.0 


55 


60 


74.7 


5.081 


0.1968 


0.3144 


23.84 


30.75 


15.05 


16.98 


375.0 


60 


65 


79.7 


5.423 


0.1844 


0.301 


24.77 


32.33 


15.76 


17.88 


389.0 


65 


70 


84.7 


5.762 


0.1735 


0.288 


26.0 


33.73 


16.43 


18.74 


405.0 


70 


75 


89.7 


6.102 


0.1639 


0.276 


26.65 


35.23 


17.09 


19.54 


420.0 


75 


80 


94.7 


6.442 


0.1552 


0.267 


27.33 


36.6 


17.7 


20.5 


432.0 


80 


85 


99.7 


6.782 


0.1474 


0.2566 


28.05 


37.94 


18.3 


21.22 


447.0 


85 


90 


104.7 


7.122 


0.1404 


0.248 


28.78 


39.18 


18.87 


22.9 


459.0 


90 


95 


109.7 


7.462 


0.134 


0.24 


29.53 


40.4 


19.4 


22.77 


472.0 


95 


100 


114.7 


7.802 


0.1281 


0.232 


30.07 


41.6 


19.92 


23.43 


485.0 


100 


105 


119.7 


8.142 


0.1228 


0.2254 


30.81 


42.78 


20.43 


24.17 


496.0 


105 


110 


124.7 


8.483 


0.1178 


0.2189 


31.39 


43.91 


20.9 


24.85 


507.0 


110 


115 


129.7 


8.823 


0.1133 


0.2129 


31.98 


44.98 


21.39 


25.54 


518.0 


115 


120 


134.7 


9.163 


0.1091 


0.2073 


32.54 


46.04 


21.84 


26.2 


529.0 


120 


125 


139.7 


9.503 


0.1052 


0.202 


33.07 


47.06 


22.26 


26.81 


540.0 


125 


130 


144.7 


9.843 


0.1015 


0.1969 


33.57 


48.1 


22.69 


27.42 


550.0 


130 


135 


149.7 


10.183 


0.0981 


0.1922 


34.05 


49.1 


23.08 


28.05 


560.0 


135 


140 


154.7 


10.523 


0.095 


0.1878 


34.57 


50.02 


23.41 


28.66 


570.0 


140 


145 


159.7 


10.864 


0.0921 


0.1837 


35.09 


51.0 


23.97 


29.26 


580.0 


145 


150 


164.7 


11.204 


0.0892 


0.1796 


35.48 


51.89 


24.28 


29.82 


589.0 


150 


160 


174.7 


11.88 


0.0841 


0.1722 


36.29 


53.65 


24.97 


30.91 


607.0 


160 


170 


184.7 


12.56 


0.0796 


O.I657 


37.2 


55.39 


25.71 


32.03 


624.0 


170 


180 


194.7 


13.24 


0.0755 


0.1595 


37.96 


57.01 


26.36 


33.04 


640.0 


180 


190 204 . 7 


13.92 


0.0718 


0.154 


38.68 


58.57 


27.02 


34.06 


657.0 


190 


200 214.7 


14.6 


0.0685 


0.149 


39.42 


60.14 


27.71 


35.02 


672.0 


200 



COMPUTATIONS IN AIR-COMPRESSION 45 

table are of course different forms of the same thing, the pressure 
to which the air is compressed. The last column of the table is 
also the same as the first merely for the convenience of following 
the lines of figures. The first column gives the pressures as 
they would actually be shown by a steam- or pressure-gage. It 
would be the actual available working pressure of the air after 
compression. The second column, or the absolute pressure, is 
obtained by adding the normal atmospheric pressure, 14.7 lb., 
to the gage pressure. The third column, showing the pressure 
in atmospheres, is obtained by dividing the absolute pressure 
by the normal atmospheric pressure, 14.7 lb. 

Column 4 gives the volume of air (the initial volume being 1) 
after isothermal compression to the given pressure; that is, 
assuming that the temperature of the air has not been allowed to 
rise during the compression, or that, if the air has not been com- 
pletely cooled during the compression, it has been cooled to the 
initial temperature after the compression. In this case the 
volume is assumed to be inversely as the absolute pressure, which 
is very nearly correct. The figures in column 4 are in fact re- 
ciprocals of tho§e in column 3, and they are obtained by dividing 
1 by the several successive values in column 3. Thus, for a gage 
pressure of 50 lb., the volume by isothermal compression should 
be 1-^4.401 = 0.2272, as given in column 4. The compressed 
volume while in the compressing cylinder, or at the moment of 
discharge, will always be greater than given in column 4 for the 
corresponding pressure, because it is impossible to compress air 
and at the same time abstract all the heat of compression from 
it. This column does, however, give the volume of air that will 
be realized if the air is transmitted to some distance from the 
compressor, or if it is allowed to give up its heat in any way 
before it is used. Air will be found to lose its heat very rapidly, 
and this column may be taken to represent the volume of air 
after compression actually available for the purpose for which 
the air may have been compressed. 

Column 5 of the table gives the volume of air at the completion 
of the compression, assuming that the air has neither lost nor 
gained any heat during the compression, and that all the heat 
developed by the compression remains in the air. This column 
shows the air more nearty as the compressor actually has to 
deal with it. In any compressor the air will lose some of its 
heat during the compression, and the air is never as hot during 



46 COMPRESSED AIR PRACTICE 

the compression nor at the completion of the compression as 
theory says it should be. The theory is all right but the air does 
lose some of its heat, as is evidenced by the heating of the cyUnder 
and the necessity of water-j acketing. The slower the compressor 
runs, within reasonable limits and other things being equal, the 
better chance the air has to give up some of its heat, 
consequently the smaller will be its volume all through the 
operation, and the less will be the power required for the compres- 
sion. 

Column 6 gives the mean effective resistance to be overcome 
by the air-cylinder piston in the stroke of compression, assuming 
that the air throughout the operation remains constantly at 
its initial temperature — isothermal compression. Of course the 
air never will remain at constant temperature during compression, 
and this column remains the ideal to be kept in view and striven 
for but never more than approximated in practical operation. 
Column 7 gives the mean effective resistance to be overcome by 
the piston for the compression stroke, supposing that there is no 
cooling of the air during the compression — adiabatic compression. 
As we have seen, there is more or less — generally less, but always 
some — cooling of the air during its compression, so that the actual 
mean effective resistance will always be somewhat less than as 
given in this column; but for computing the actual power re- 
quired for operating air-compressor cylinders the figures in this 
column for the given terminal pressures may be taken and a 
certain percentage added for friction — say 5 per cent. — and the 
result will represent very closely the power required by the 
compressor. In proposing to add 5 per cent, for friction we do 
not mean that the total friction of a steam-actuated air-compressor 
will be only 5 per cent., for it will probably be more than 10 per 
cent., but part of this 10 per cent, will have been conpensated 
for by the partial cooling of the air during the compression. 

The values given in columns 6 and 7 are used in computing 
the horse-power of an air-compressing cyhnder precisely as the 
mean effective pressure per stroke in a steam-cylinder is used in 
computing its power. In the steam-cylinder the computation 
gives the power developed by the steam, and the same system 
of computation applied to the air-cylinder gives the power used 
in the compression. 

y Having an air-compressing cylinder 20 in. diameter X 2 ft. 
stroke at 75 r.p.m., or 300 ft. piston speed, compressing air 



COMPUTATIONS IN AIR-COMPRESSION 47 

adiabatically to 75 lb., we find in column 7 that the mean effect- 
ive pressure is 35.23 lb., and then the horse-power required 
will be computed as follows: 

202 X 0.7854 X 35.23 X 300 ^ 33,000 = 100 h.p. 

The mean effective pressures given in columns 6 and 7 being 
for compression to different gage pressures from an initial pressure 
of 1 atmosphere, it does not follow that those values will be 
correct for computations in compound compression, or for com- 
pression from any other initial pressure but that of 1 atmosphere. 
Thus in column 7 the M.E.P. for compressing from 1 atmosphere 
to 50 lb. gage pressure is 27.39. In this case the pressure of the air 
compressed is increased 50 lb., but it does not follow that we can 
take air at 50 lb. and compress it to 100 lb. with the same mean 
effective pressure. In the latter case the M.E.P. required would 
be 40.33, or 47 per cent, greater than in the former case. 

Column 8 gives the mean effective resistance for the compres- 
sion part only of the stroke in compressing air isothermally from 
a pressure of 1 atmosphere to any given pressure. This at once 
calls our attention to the two distinct operations involved in 
practical air-compression: the actual compression of the air 
to the given pressure, and the delivery or expulsion of the air 
from the cylinder after the full pressure is attained. These 
two operations correspond inversely to the two operations occur- 
ring in the cylinder of a steam-engine: the admission of the steam, 
where it is sustained at approximately full pressure until the point 
of cut off, and the expansion of the steam from the point of cut 
off to the termination of the stroke, the expansion period in the 
steam-cylinder corresponding inversely with the compression in 
the air-cyUnder, and the admission of the steam corresponding 
with the delivery of the air. 

It will be noticed that the mean effective pressures in columns 
8 and 9, for the compression part only of the stroke, are much 
lower than those in columns 6 and 7 for the whole stroke, but 
when to the work of the compression part of the stroke is added 
the work of delivery, the values will be found to correspond very 
nearly. Thus when compressing adiabatically to 50 lb. gage 
pressure the volume of air delivered will be (column 5) 0.35 of 
the original volume, or 0.35 of the stroke for each cylinderful of 
free air, so that the pressure or resistance for 0.35 of the stroke 
will be 50 lb., while for the compression part of the stroke, 1 — 



48 COMPRESSED AIR PRACTICE 

0.35 = 0.65, the resistance will be 15.05, as given in column 9. 
Then (15.05X0.65) + (50X0.35) =27.28, which corresponds as 
well as could be expected with the value in column 7 for the whole 
stroke, 27.39. 

There is also to be observed a less proportional difference 
between the values in columns 8 and 9 than between those in 
columns 6 and 7, but this also will be found to be compensated 
for by the differences in terminal volume for isothermal or for 
adiabatic compression and the different proportion of the stroke 
occupied by the full pressure of delivery. Thus comparing the 
figures for isothermal compression with those just given for 
adiabatic compression, compressing to 50 lb., as before, we have: 
(13.48X0.7728) + (50X0.2272) =21.78, a result which may be 
said to be identical with the value 21.79 for the whole stroke, as 
given in column 6. 

Columns 8 and 9 may be found serviceable in some cases in 
computing the power used in the first stage of compound com- 
pression, where generally the entire function of the first cylinder 
is that of compression only, its total contents from the beginning 
to the end of the stroke being simply compressed into the volume 
contained in the smaller cylinder, and there being no part of the 
stroke properly occupied in delivery or expulsion at any com- 
pleted pressure. 

Column 10 gives the theoretical temperature of the air after 
compression, adiabatic, to the given pressure. As we have re- 
marked elsewhere, the actually observed temperature in these 
cases is never as high as the theoretical temperature. This is not 
that the theory is incorrect, for, as usual, the theory is more 
nearly correct than ''practical" people are wont to allow. If 
the temperature of the compressed air by observation is not 
found to correspond with the figures as given, it is only because 
the air is being cooled by conduction or radiation even while it 
is being heated by compression. 



CHAPTER V 
THE INDICATOR ON THE AIR-COMPRESSOR 

The steam-engine indicator, so called from the incident of its 
inception, is just as much an air-compressor indicator, and it 
can tell us just as much, or perhaps a little more, of what goes 
on in the air-cylinder as it tells us of the steam-cylinder. All 
the conditions seem specially to invite the application of the 
indicator to the air-compressor, and to the study of air-com- 
pression practice and results by its aid. In fact the air-com- 
pressor seems to be the ideal and only perfect field for the indi- 
cator. A steam actuated air-compressor may be said to be the only 
machine where an indicator can be applied and be made to tell 
the whole story of the power developed and of the work actually 
done. 

In the steam pump of any type the report from the card of 
the water cylinder is affected by questions relating to the inertia 
of the body of water. With a steam-engine and the bare testi- 
mony of the indicator-card, there is always some uncertainty 
about the friction of the working parts of the engine. We may 
take what we are pleased to call the ''friction diagram," when 
the engine is running without doing any external work, and we 
know what resistance the steam has to overcome at that time; 
but that tells us comparatively little of the resistance of the 
engine parts when loaded. We know that the friction of nearly 
every working part of the engine increases with the load, but, 
when the load is on, we do not know from the indicator-card how 
much of its mean effective indicates actual work done, or how 
much of it belongs to the friction of the engine, and to get the 
result with any certainty and accuracy it is necessary to employ 
some form of dynamometer in connection with the indicator, 
and let them fight it out between them. 

In the case of the air-compressor this is all different. The 
air-compressor is its own dynamometer. By taking cards from 
both the air- and the steam-cylinders at the same time, or when 
the compressor is running under the same conditions, we get a 

49 



50 COMPRESSED AIR PRACTICE 

perfect statement of the power developed and of the actual 
work done, and then we know, too, that the difference in indi- 
cated horse-power between the air- and the steam-cards clearly 
shows the power that has been expended merely to keep the ma- 
chine in motion. The cards not only give the comparative 
total power and work, but also the relations of the one to the 
other at any point of the stroke, showing the air resistance at 
any point, as well as the force of the steam at the same point, 
and through this knowledge it will advise us whether the air is 
compressed with economy or whether better results are to be 
sought for. 

Realizing the importance of the indicator as an indispensable 
aid in the full development of economical air-compression, it 
is proper that we learn what we can of the peculiarities of the 
air-card and of the means of manipulating and interpreting it. 
We can only consider at first the card from the single air-cylinder, 
in which the whole operation of air-compression is completed 
at a single stroke. The cards from cylinders in which either 
stage of a compound compression is carried on assume peculiar 
shapes, which we may find pleasure in studying later on. 

To an indicator-man who has been brought up, as most have, 
exclusively upon steam-cards the air-card is at first a little con- 
fusing, from the fact that all the operations upon the one card 
are the reverse of those upon the other. The admission-line of 
the steam-card is the delivery-line of the air-card; the expansion- 
line in the one is the compression-line in the other; the exhaust or 
back-pressure line is the admission-line, and the compression- 
line becomes the re- expansion-line. One can, however, soon 
''catch on" and become familiar with each operation and the 
way it is shown by the lines of the diagram. 

It is not the purpose of this work to instruct in the application 
and use of the indicator. We must assume that it is in competent 
hands, or its evidence will be worthless. Indicator-cards have, 
however, a way of telling for themselves frequently if they have 
not been taken with a reasonable regard for the essential con- 
ditions. As the peculiarly important part of the air-card is the 
compression-line, it is necessary that the drum movement be 
correct, and that, in proportion to its length, the travel of the 
card shall be accurately coincident with the piston travel at all 
points. Cards, to be relied upon, should not be taken until the 
compressor has been run long enough to have attained its com- 



THE INDICATOR ON THE AIR-COMPRESSOR 51 

plete working conditions. We know that the compression of 
air heats it, and that the heat then in the air is communicated 
more or less to everything in contact with it. When the cylinder 
becomes heated, it has its effect back again upon the air, and 
until the compressor has been run continuously and at full 
pressure for an hour or so, the full temperature of the working 
parts has hardly been reached, and the effect of the heated parts 
upon the temperature of the air at different points of the stroke 
will not be correctly indicated. Cards taken from a compressor 
that has only just been started will give a lower compression- 




FiG. 8. — Theoretical Indicator Card. 



line and a lower mean effective pressure (M. E. P.) than those 
taken after the cylinder and piston and connecting parts have 
been heated up to their mean working temperature. 

Fig. 8 is offered as an ideal and typical single-compression 
air-cylinder card, designed to show the points and properties of 
the card, and the methods of manipulating and studying it. 
The card is somewhat smoother and cleaner and in most respects 
more perfect than any actual card, except that the admission- 
line is purposely drawn rather low to keep it perfectly distinct 
from the atmosphere-line. The lines constituting the actual 
diagram are as follows: 



52 COMPRESSED AIR PRACTICE 

AB, Compression-line 

BC, Delivery-line 

CD, Re-expansion-line 

DA, Admission-line 
These constitute the actual card, and together represent the 
complete cycle of operations occurring in one end of the air- 
cylinder for one complete revolution of the compressor-crank. 
The atmosphere-line, MN, also is traced by the indicator, and 
is the neutral line of the diagram, or the line of departure in air- 
compression. 

For the proper interpretation of the diagram additional lines 
are to be drawn as follows: EF, the line of perfect vacuum. 
This line is drawn parallel to the atmosphere-line, MN, and at 
a distance below it determined by the scale of the diagram. 
The pressure of the atmosphere at sea-level being 14.7, and 
always decreasing as the altitude increases, the practice of 
calling the atmospheric pressure 15 lb. may be said to be a 
rather loose one. If the compressor is operated at a considerable 
altitude above the sea-level, as many are, the atmospheric pres- 
sure at the time and place where the diagram is taken should be 
ascertained by a barometer, and the line EF be drawn accord- 
ingly. It should be remembered, as we will see when we get to 
it, that a height of only a quarter of a mile, or a little over 1300 
ft. will make a difference of 7 per cent, in the volume of air 
delivered. 

The vertical lines PA and CL having been drawn perpen- 
dicular to MN, defining the extreme length of the actual diagram, 
the clearance-line GH may next be drawn. This is drawn 
parallel to CL, and the distance CG or LH may be ascertained 
by computation as follows: The volume represented by the 
rectangle APCL is the actual displacement of the piston for its 
whole travel. The volume of air acted upon by the piston is 
this volume increased by the volume CGHL remaining in the 
clearance-space of the cylinder. This volume of air, CGHL, 
at the end of the compression-stroke, and at the pressure in- 
dicated by the diagram, has upon the return stroke of the piston 
re-expanded until it reached the atmospheric pressure again at 
D. This re-expansion is so quickly accomplished that whatever 
the temperature at the beginning the re-expansion is practically 
adiabatic. The relative volume before and after the re-expan- 
sion may be found in column 5 of Table VII. Assuming the 



THE INDICATOR ON THE AIR-COMPRESSOR 53 

scale of the diagram to be 30 and the pressure at CG to be 70 lb. 
gage, and designating LH by x, we have the proportion 

x:DL-\-x::0.2S8:l 

Then the length DL being 0.25, in. we have 

x:0.25-\-x: :0. 288:1; 
then 

a: = 0.072+0. 288a:, 
and 

0.712a; = 0.072, 
a; = 0.101. 

So that CG or LH equals say yV in., and GH may be drawn 
accordingly. 

Having drawn GH, the rectangle APGH represents the total 
volume of air subjected to compression for the stroke, and 
noting the point a, at which the compression-line begins to rise 
from the atmosphere-line, and drawing the perpendicular ae, 
then aeGH represents the total volume of air at atmospheric 
pressure. The point a, being the point at which compression 
from atmospheric pressure begins, may be considered the begin- 
ning of the whole diagram, and the cycle of operations for the 
entire stroke may be considered to start from this point. 

For computing the mean effective resistance the entire en- 
closed area of the actual diagram ABCD is to be taken, and 
this area may be measured by the planimeter, or by the mean 
of a series of ordinates in the customary way, as with any other 
diagram. The area lying below the atmosphere-line of course 
represents the resistance upon the return stroke, but the diagrams 
from both ends of the cylinder being assumed to be similar, the 
entire area may be taken for the single stroke. The correct 
practice is to take diagrams from both ends of the cylinder, 
and it should be followed if possible, but it is clearer and simpler 
for us here to consider only the single diagram. 

The M.E.P. of the diagram having been ascertained, the 
indicated horse-power (i.h.p.) represented may be computed 
precisely as in the case of a steam-engine. Thus the M.E.P. 
in the diagram before us happening to be 30, if it were taken 
from a cylinder 20 in. diameter X 24 in. stroke at 80 r.p.m. 
the i.h.p. for the double stroke will be as follows: 

202X0.7854X30X4X80-^-33,000. 



54 COMPRESSED AIR PRACTICE 

I like always in such cases to put it down in this way, that I 
may be sure that I get in all the ingredients. It is not necessary 
to run for a table of squares or of areas, and no time is saved 
by doing so. The decimal 0.7854 is always cleanly divisible 
by the constant divisor 33,000, giving us 0.0000238 as a substi- 
tute constant for both of these combined. It is not difficult 
to remember this or to keep it posted with other labor-saving 
devices in a convenient place. The ciphers in the other factors 
will help us to elbow the decimal point to the right, and our 
case will then stand like this, a little string of simple and 
easy multiplications: 

202X0.0000238X30X4X80 = 

22X0.238X3X4X8 = 

0.238X384 = 91.39 i.h.p. 

We will not here go into the question of the additions to be 
made to this for friction, etc. 

The i.h.p. having been ascertained, that gives us the power 
consumed, or the cost of the compression, and then we naturally 
want to know the actual quantity of air compressed and delivered. 
The indicator-diagram shows this very accurately. At the 
point a, where the compression-line takes its departure from the 
atmosphere-line, the cylinder is shown to be full of air at the 
atmospheric pressure and corresponding density. This is not 
the whole cylinder, as a portion of it, Aa, has been already 
traversed by the piston. Whatever proportional distance the 
point a may be from the beginning of the stroke is to be deducted 
from the total length of the stroke, and the remainder represents 
the total actual volume of air at atmospheric pressure subjected 
to compression for that stroke. The compression and delivery 
of the air goes on with the advance of the piston until it reaches 
the extreme end of its stroke at CL, but when that is reached, 
the clearance-space LCGH is filled with air compressed, but 
not delivered, and upon the return of the piston this air re- 
expands until it reaches the atmosphere-line at o, so that prac- 
tically the travel of the piston from o to L and back again has 
accomplished nothing toward compression, and the distance 
oL also is to be deducted from the total length of the line AL, 
when that line is taken to represent the volume of air com- 
pressed and delivered. In the diagram before us, if AL be 3| 
in. and ao be 3iV in., the ratio of air compressed and delivered is 



THE INDICATOR ON THE AIR-COMPRESSOR 55 



3.4375 



= 88 per cent, of the cylinder capacity. As was remarked, 



3.875 

this card does not represent actual practice, and the ratio is 
generally not as low as this, being seldom less than 5 per 
cent, and from that up to 10 per cent, instead of the 12 per cent, 
here shown. 

In compressors having positively moved inlet valves, if the 
valve is set to open while the crank is on the center, the com- 
pressed air in the clearance space is released at once and there 
is no normal re-expansion-line but a sudden vertical drop from 
the top, and there is no means of ascertaining the amount of 
clearance from the card. 













































































r 




^ 
































/ 


/ 


/ 






































/ 


/ 


































L 


V 


/ 


1 


! 




























i^^ 


/ 


/ 




I 




























J^±/ 


> 


V 




























^ 


f/ 


r 


/a 




1 
























^^ 


/ 


/. 






























\> 


..^"^Q 


V 


p*^ 






J 














^^.-^ 






^ 


x;^ 


^ 


^^'^ 










I 




-s 




u 


^ 


^ 


n- 




== 












p 





A 
1.0 



0.4 



0.3 



0.1 



100 
90 

80 

33 

70 'B 

3} 

m 

eo| 

.00 |D 
O 

4U 
30 



iPl4.7 Lb. 
Absolute 
B 

13.7 Lb. 
0. Absolute 



Fig. 9. — Laying Out Adiabatic and Isothermal Curves. 



So far as the indicator has anything to say about the economy 
of the air compression in any given case its evidence is found 
chiefly in the compression- line of the diagram, and for comparison 
it is necessary to describe upon the diagram the theoretical iso- 
thermal and adiabatic compression curves. 

There are various ways of doing this and among the best within 
my knowledge is that devised by Mr. H. V. Conrad first 
printed in Power and here reproduced in his own words. 

"Indicator diagrams taken from air cylinders always show the com- 
pression curve as starting below the atmospheric line, when the compres- 
sor is drawing free air. This starting-point of compression may range 
from 1/4 lb., in the high-class machine, to 1 1/2 lb. or more, below the 



56 



COMPRESSED AIR PRACTICE 



o 
o 

< 
M 

H 

o 

Q 

H 
H 

O 

W 
H 

w 

Eh 

O 
O 



39 

i 

p. 

1 

o 


CO 


Tt< 00 (N CO O 


■* 0> CO 00 (N 








o o o o o 


o o o o o 


o o o o o 


o o o o o 


(6 d> <6 (6 dl 


i°iU 


CO OJ IN CO O 
00 l^ l^ CO CO 

§§§§§ 


§§s§§ 




o T}< i> i-i lo ! 

03 00 t^ l^ CO 
00 00 00 00 00 
O O O O O j 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


d do d d I 

j 


o 

§ 

o 


IN K3 t^ O 


CO t^ o CO CO 

■<1< CO CO IN —< 


05 I— 1 CO iC oo 

O O 0> 00 fe 
^ ;^ O O O 


lllll 




o o o o o 


o o o o o 


o o o o o 


o o o o o 


O O O O O j 


« lO 00 o 
00 t^ CO ic »o 
<N (N (N (N (M 


<N cq IN (N CS) 


IC t> 05 -H CO 
O Oi 00 00 l> 

Oi ^ ^ r^ ^ 


CO 00 ^ CO CO 
CD lO IC TJH CO 


1128 
1119 
1111 
1102 
1094 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


O O O O O j 


M CO O (N 
O Oi 00 00 t^ 
■^ CO CO CO CO 


■* CO l> 00 o 


IN CO IC CD CO 

csi ^ o a> 00 

CO CO CO IN IN 


00 O CO Tj< iC 


CO CO CO CO CO 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


g 


(N IC lO CO 
lO Tt< CO (N ^ 
U3 lO lO >0 lO 


X O ^ (N IN 
O O 0> 00 t> 
lO lO •* -* Tf< 


21 ^ ^ ^ ^ 


2§§gg 

>* Tft CO CO CO 


O .-< CO ^ .-< ^ 

CO CO CO CO CO 


o o o o o 


o o o o o. 


o o o o o 


.o o o o o 


o o o p o 


o 


t^ t^ r* t^ CO 


CO CD CO t^ 00 
00 t^ CO lO ■* 
CD CO CO CO CD 


00 t^ CO CD CO 
CO IN rH O Oi 

CO CO CO CO >o 


CO lO lO CO CO 


i2 J2 2 2 2! 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


o o o o o 1 


o 


r* CO lo CO (N 

G^ O O^ O^ O^ 


lO IC IC Tf< CO 
0> Oi 00 00 00 


(N O 00 CO TJH 
CD »0 CO IN 1-1 

00 00 00 00 00 


CO CO O O 00 
O 05 00 t' lO 
00 l> t> t^ t^ 


t^ >0 »0 CO ^ 

rt* CO CO rH O 

t^ t>. i> I- t^ ! 

j 


d d d d d 


o o o o o 


o o o o o 


o o o o o 


o o o o o ' 


O 

o 


(N (M (N (M (N 


'-I O 05 t^ CO 
<N IN -H rH ^ 

<N Cq (N (N IN 


(N IN Cq IN (N 


O CO <N iC 

§&§§§ 

IN <N <N CO CO 


O O O OS OS i 
CO CO CO 1-1 '1 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


66660^ 


lO 00 »o 

OS t^ CO T)H CO 

(N M S M ^ 


(N lO 00 CO 

(N i-i O t^ CO 


o r^ c^ o ic 

(N (N CM IN (N 


in CO t^ Tt< 

00 CD lO CO CO 


ICi 00 »c 
1-1 OS 00 CO "5 

Tt< CO CO CO CO 

CO CO CO CO CO 


O O O O O 


o o o o o 


o o o o o 


o o o o o 


0000c 


o 
o 
o 


CO CO CO CO CO 


<-< O 00 CD >0 
C^, OJ ^ ^ ^ 

CO CO CO CO CO 


CO IN O CJS 1> 
CO CO CO CO CO 


CO 

CO CO CO CO CO 


IC CO IC 

l^ CD Tj< CO --^ ' 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


6 6 6 6 6\ 


T}f C<l O 00 t^ 

(N (N (N rH rH 


CD -^ IN rH 05 
,-1 _ _( r-l O 
•* T}< TJH Tj* Tj< 


Tf< 1* ■<}< -^ Tj< 


00 CO ■* IN O 
05 03 C» 05 0> 
CO CO CO CO CO 


CO lo t^ I 

OS r* "5 CO ^ 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


00000 


+ + 

iC CO <N ^ Oi 
§ § § g § 


t^ lO CO IN O 
U3 ic lO »0 lO 


00 t-- O CO 1-1 

t^ t^ t^ t^ t^ 

lO »0 lO lO o 


05 00 CD ->!< CO 


00 CD Tt< CO 

CO lo lO 10 »c 
lO »0 lO 10 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


00000; 


+ 

CO >C CO CO (N 

rf< Tf Tjt -^ Tf< 


r-l O 00 CO -^ 
Tf< T}H CO CO CO 

l> o t- t- t- 


CO IN O 00 CD 
CO CO CO CO IN 
O l> I> t- t- 


ggggs 


t^ CD Tf CO CO 

t^ n! t^ t^ t^ 


O O O O O 


o o o o o 


o o o o c 


o o o o o 


00000 


+ 1 
iC ■* (N IN iM 

00 00 00 X 00 


-1 O 05 00 t^ 
00 00 00 00 « 


CO lO -^ CO c^ 
00 00 00 00 00 


tH O 05 00 t* 

■<# •* CO CO CO 


00 00 00 00 00 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


00000 


•11 

•" a 

J 1 
la 


t^ CO U5 -* CO 


(N ^ O 0> 00 


l^ CO »0 ■* CO 


CO 1-1 O OS 00 


t^ CO 10 Tt< CO 


i 


Tf •* Tj< CO CO 


CO CO CO CO CO 
1-H rH i-H fH rH 


CO CO CO CO CO 


IN CO CO CO CO 



THE INDICATOR ON THE AIR-COMPRESSOR 57 









(N 


t* 


^ 


o 


o 


lO 


05 CO 


r^ 


^ 


lO 


a> 


^ 


on 


CM 


o 


i-< 


m 


n 


CO 


00 CM lO 






o 




























t^ 




















r^ 


r^ 




t^ 


r^ 


t^ N- 




N. 






















§SS| 








O 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 








o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o , 






00 


(N 


r^ 


o 


CO 


CO 


CO 


^ 




^ 


^ 


I-l 


Tfi 


on 


^ 


»o 


nn 


^ 


lO 


00 1-1 »o 1 






o 






•* 


■<*< 




CM 


CM 1-t 


o 




05 


on 


on 




CD 


CO 


lO 






CO 


S Cq r^ 






00 


« 


00 


00 




00 


00 00 




00 
















1^ 


t^ 










o 


O 


o 


o 


o 


o 


o o 


o 


o 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


■^ 
p 






o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 




00 


<N 


lO 


Ni 


o 


CO 


CO t^ 


05 


CM 


»o 


1^ 




CM 


IC 


l^ 




CM 


Tt< 




C3S -1 Tt< 






o 










r^ 




ITS TJH 




CO 


CM 






o 
















1 








05 




03 




C5 05 


OS 


Oi 


Ol 


{-» 


rrs 


m 






on 




00 


00 00 00 




^ 


o 


O 


o 


o 


o 


O 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


Y 






o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


H 


CD 


00 


o 


w 


>o 


00 


CM 


^ 




^ 


rr, 


CM 




CD 


00 




^ 




lO 


CO 00 


^ 




§ 




1^ 


r^ 








Tf< CO 


























CM ^ C 




o 






o 


o 


o o o 


o 


o 


o 


ct> 


05 


Hi 


OJ 


C5 


05 


rT3 


n 


OS 


OS OS OS 


^q 








'-I 


»-• 


•-< 




l-H 1— I 


i-H 




T-l 


O 


o 


O 


O 


o 


o 


o 


o 


o 


o o c 


Z 




— 


o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


o 


(N 


lO 


h- 


on 


ffl> 




CM CO 


i^ 


CO 


on 


fT> 


^ 


J_, 




^ 


^ 


•^ 


in 


fO 


t>. 00 


OQ 






05 






CO 


»o 


>c 


Tt* CO 




o 


<li 


<Ti 


00 


o 


CO 


in 


s 


CO 


CM 




OQ 








I— 1 


i-H 












I-l 




o 


o 


o 


t^ 


o 


o 


o 


o 


o o o 


Ah 
1^ 






d 


d 


d 


d 


d 


d 


d d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


<6 (6 c> 


<N 


Tt< 


«o 


»o 


lO 


CO 


CD 00 


m 


rh 


m 


n> 




















8 










o 


S 


00 


r^ 


CO ic 


i-i 


CO 


CM 




,_( 


o 




00 






»o 




CO CM 1-1 




00 


CO 


CO 


CO 


CM 


CM 

1-1 


CM CM 

r-( t-l 


^ 


CM 


CM 


CM 


CM 
















!^ 


^ 




d 


d 


d 


d 


d 


d 


d d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


6 6 6 


^ 


:3 




IC 


■* 


CO 


(N 


^ 


^ 














on 


lO 


lO 


iC 


lO 


lO 


CO 


CM ^ 


tf 
H 





o 


00 


t^ 


CO 


lO 


•* 


CO 


CM .-1 


o 


o> 


on 


r^ 


CO 


M* 


CO 


CM 




o 


en 


nn 


t^ CO in 




■* 


T** 


■* 


-* 


■* 


M< 


■* Tt< 




CO 


CO 


CO 


CO 


CO 


CO 


CO 


CO 


CO 


CM 


CM 


CM CM CM 














































15 






d 


d 


d 


d 


d 


d 


d d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


6 6 6 


i 












































m 






a> 


r^ 


CO 


rt< 


CM 


CM§ 


on 


lO 


Tt< 


Tl< 




05 


IC 


00 








lO 


+ 






(—1 




r» 






Tf< 


CO 


05 


00 


r>. 


CD 


1(5 










ffli 




CO 






2 




«o 


<© 


CD 


CO 


CO 


CD 


CD CO 


iC 


u:. 


iC 


lO 


lO 


lO 


IC 


IC 


lO 


T}i 


'^ 


^ 


^ ■* Tt< 


p. 




'"' 


'"' 


""* 


'"' 


'"' 


""• 


r-l rH 


'~' 


*"• 


'"' 


'"' 


""^ 


'-' 


'"' 


'"' 


*"• 


'"' 


'"' 


'"' 


r-l rH I-l 


1 




o 


o 


o 


o 


o 


O 


O O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 




^ 


00 


CO 


lO 


o 


lO 


•* CM 




»o 




on 


ID 




lO 




1 


lO 




lo 


lO + CD 


o 


<o 


•^ 


CO 


(N 




o 


00 t^ 


CO 


Tt< 


CO 




O 


o> 


N. 


CO 


lO 


CO 


CM 


o 


OS 00 CD 


o 




>o 


05 


Oi 


05 


a> 


a> 


00 


00 00 


00 


00 


00 


00 


00 




^* 


tN. 


t> 


t^ 


t^ 


tN. 


CD CO CD 




- 


d 


d 


d 


d 


d 


d 


d d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


d 


6 6 6 


w 


00 


^ 


CO 


(N 


00 


CM 


lO 




lO 




lO 




CM 










lO 








o 




(N 


o 






CO 


»0 CO 


CM 




OS 




CD 




CO 




o> 


00 




lO 




H 




00 


CO 


CO 




CM 


CM 


to e^ 


CM 
















o 


o 




o 


o o o 






(N 


<N 


(N 


CM 


CM 


CM 


(N (N 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 


C^ CM CM 








o 


O 


O 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


O 


O 


o o o 








(N 


r^ 


(N 


^ 


lO 




in 


00 




s 


in 


in 


lO 








+ 


1 








o 

CO 


fJJ 


r^ 


lO 




CM 


o 


a> t* 


lO 


CO 


CM 


00 


CO 


-a^ 


CO 


I-l 


05 


b- 


CO 


SSiO 


O 
O 






on 


on 


00 


00 


t* t>. 


S:3 


r* 


r^ 


t^ 




















N 


<N 


CM 


CM 


CM 


M 


CM CM 


CM 


CM 


CM 


(N 


CM 


CM 


CM 




CM 










- 


o 


O 


O 


o 


o 


o 


o o 


o 


o 


o 


o 


O 


o 


O 


o 


o 


o 


O^ 


o 


OOP 

1 






in 


«o 


»o 


CM 




















+ 






g 


OJ 


r^ 


lO 






m 


r^ to 


CO 


i-H 


05 


t^ 


lO 


CO 


I-l 


a> 


CO 


rfi 


CM 




00 lO CO 




r«. 


r^ 


r^ 


1^ 


l> 


CO 


CO CO 


CO 


CD 


to 


lO 


lO 


lO 


iC 


•^ 


■^ 


rfi 


Tt< 


■^ 




hH 




M 


CO 


CO 


CO 


CO 


CO 


CO CO 


CO 


CO 


CO 


CO 


CO 


CO 


00 


CO 


CO 


CO 




CO 


CO CO CO 


3 






O 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 




















+ 


1 




in 


















Pi 




o 


o 


00 


CO 


r»4 


CM 


o 


00 CO 


CM 


o 


o> 


t>. 




CM 


o 


00 


«o 


IN 


o 


00 


o o o 




»o 










s 


CO CO 


CO 


CO 


CM 


CM 


CM 


CM 












£ 






»o 


to 


lO 


iC 


»c 


»o »o 


»o 


« 


lO 


»o 


lO 


lO 


to 


to 


»o 


lO 


lO 


lO 


tC IC lO 




- 


o 


o 


o 


O 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o_ 


o 


o o o 


<J 




















in 
















2 

1 








a> 


t^ 


lO 


CO 


,_l 


05 t^ 


to 


CO 




o 


on 


CO 


'^ 


CM 


o 


00 


CO 


•* 


CM OS CO 






fH 


o 


o 


o 


o 


o 


Oi 05 


05 


o 


Oi 


05 


00 


00 


00 


00 


00 


h- 


h- 










t» 


t^ 


t^ 


t>. 


t^ 


t^ 


CO CD 


CD 


CD 


CO 


CD 


CD 


CD 


CD 


CO 


CO 


CD 


CD 


o 


CO CO CD 




- 


o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


1 
X 












lO 


lO 




»o 




t>. 


CO 


CO 






+ + 










U5 


o 


^ 


00 


^?5 


;^ 


^^ 


^ 


00 


t^ 


iO 


■* 


CO 


CO 




o 


o 


fe 


o 


S§o 


f^ 




(N 


w 


00 


00 


w 


00 


00 


w « 


« 


00 


00 


00 


00 


00 


00 


00 


00 


00 


w 


00 


00 » 00 


S 

3 






o 


o 


o 


o 


o 


o 


o o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o o o 


i| 












































3 


w 


1-1 


o 


OS 


00 


r* 


CO to 


Tj< 


CO 


CM 


^ 


o 


OS 


00 


r* 


CO 


»o 


■«*« 


CO 


CM -H O 




li 


IN 


(N 


(N 


^ 


^ 


^ 


r^ ^ 


^ 


^ 


^ 


^ 


^ 


o 


o 


o 


o 


o 


o 


o 


6 6 6 








"■ 




"■ 


































S ^ 












































< 


a 











































58 



COMPRESSED AIR PRACTICE 



1 


1 

0. 
d 

i 
i 

1 

O 


o 
o 
o 

o 

00 


+ 

O OS 00 t^ <0 

00 t^ t;^ t;^ r^ 


1 + 1 + + 

<D lO ira CO (N 

t- t^ t- t^ t- 


+ 1 +1 


1 ++ + 

00 t^ CD to Tj< 

CD CD CD CO CD 


+ 1 1 

CO CO <N ^ 

CO CO CO CD CD 




o o o o o 


o o o o o 


o o o o o 


00000 


00000 




c.- + + 

t^ «D lO ■>* CO 
0> 05 05 05 05 


1 + + + 

CO CM ^ O O 
Oi O Ci O 00 


+ + 1 1 

00 t- l^ CO >c 


1 + 

•<*< CO (N ^ 

00 00 00 00 00 


1 1 1 1 

OS 00 t^ CO 
00 t^ t^ t^ h- 




o o o o o 


o o o o o 


o o o o o 


00000 


00000 


« 


a> 00 t^ CD lo 

Cil CM (N (N (N 


+ + + + 1 

Tj( CO W -H rH 

(N <N (N (N (N 


1 1 1 1 1 


+ + + 1 

IN ° S ° W 


OS 00 CO 10 

OS 2 2 S 


o 


o o o o o 


o o o o o 


o o o o o 


00000 


00000 


+ + + 

W (N -^ O 00 
CO CO CO (N (N 
N N CM M (N 


+ + 1 1 

(N (N (N (N <N 


+ + 1 1 1 

(N rH ^ O OS 
IN Ol (N (N -H 
(N (N (N IN (N 


+ + + 

r^ CO »o rt< CO 

M (N (N (N IN 


N IN W N CS| 


Oh 
1^ 


o o o o o 


o o o o o 


o o o o o 


00000 


00000 


+ 

00 t^ <0 lO Tj< 
M (N N N C^ 


CO <N r-l O OS 

Tf Tt< rt< Tf CO 
M (N (N (N (N 


+ + 

00 t^ to Tt< CO 
00 CO CO CO CO 
(N (N (N (N (N 


+ + 

(N -< 00 h- 


+ 4- + u^ 

CD UO -"t CO CM 
CM "N N CM C>3 


8 


o o o o o 


O O O O O 


o o o o o 


00000 


00000 


< 

S 

o 


+ 

<© »0 •* CO <N 


1 1 1 
^ O OS 00 t^ 
O CD lO IC »C 
Dl (N (N <N (N 


+ 1 + + 

lO Tt< 00 ^ O 

'^ s ?i5 ^ s 


+ + 1 + 

OS 00 00 CO >o 


1 1 

■* CO -1 OS 


o o o o o 


o o o o o 


o o o o o 


00000 


00000 


o 


+ 1 

W t- t, lO ■* 


1 + + + 

00 ^ O OS 00 


+ + + + 

CD IC M* 00 (N 


+ 1 

OS 00 r^ CO 


tH CO CM OS 
W W M N c5 


o o o o o 


o o o o o 


o o o o o 


00000 


00000 


§ 


1 + 

>0 TJ4 00 (N O 
00 CO CO CO CO 


CO CO CO 00 CO 


+ 1 + 1 

00 CO 00 (N (N 


+ + + 

CO »0 CO IN --1 


1 + 10 

00 t^ CO T|« 


d d> d> <D d 


o o o o o 


o o o o o 


00000 


6 6 6 6 6 




o 

»o 

o 
o 

CO 

I 

;g 

1 
1 _ 

o 
tti 


+ 

OS 00 t^ lO Tt< 

Tt< Tt< Tt< Tf< Tt< 

CO CO 00 CO CO 


+ + 

(N ^ O 00 b- 
rt< Tt< ■>*< 00 00 

CO CO CO CO CO 


+ + 1 

CO 00 00 CO CO 


00 00 CO CO CO 


1 + 

CM OS 00 t» 
CM CM — 1 —1 >-i 
00 00 00 00 CO 


H 

§ 


o o o o o 


o o o o o 


o o o o o 


00000 


00000 


00 CO 00 CO CO 


1 + 1 + 1 

N. lO Tf (N f-H 

CO CO 00 CO CO 


+ 1 + + 

E' g? Sr i? i? 

5:5 ?;; 5;; ?:; 5;; 


+ + 

t^ CO CD CO CD 
CO CO CO 00 CO 


+ 1 

00 00 00 00 00 


o o o o o 


o o o o o 


00000 


00000 


00000 


■^ 00 (N O 00 
W IC »0 IC 00 

Tj< Tjt T}< Tt T}< 


1 

t^ iC Tf< (N ^ 

•^ Tf< T}< Tjt Tt< 

^ ^ ^ ^ ^ 


+ + 

OS r^ CO Tf CO 


+ i JJ5 
CO §1 IN W IN 

Tt< T}< -^ Tf rt< 


§Si55 


^ 


1 o o o o o 


o o o o o 


00000 


00000 


00000 


2 

g 


CO <N ^ OS W 
Tt ^ rj* CO 00 
lO U5 W5 lO »0 


lO »0 «? »0 U5 


+ 

00 b- iC CO Ol 

S S Ss s s 


+ 1 

00 CO 10 00 

10 »o 10 »o "? 


in ui ici ui to 


1 d d d d d 

' (N ^ O QC o 

1 OS O 05 00 » 
50 O CD O O 


o o o o o 


00000 


00000 


00000 


< 


+ 1 

l-O CO N -< O 


1 

00 t^ CO ■<*< (N 

t^ t^ r^ r- (>- 

CD CD CD CD 


1 + + 

r^ t^ 10 ■* 


+ 1 

CM -H 00 CO 


W 


1 d d d d d 


o o o o o 


00000 


00000 


00000 


1. 
>< 


+ 

N rH -H O g 
W W 00 W 00 


|||S| 


+ + 1 

« 00 « t» t^ 


+ 


1 1 + 

r^ b. t^ r* t- 


s 


o o o o o 

1 + 

U5 lO T}. Tt< CO 


o o o o o 


00000 


00000 


00000 


1 

c<i ^ o OS OS 

00 « 00 00 00 


1 1 + 

00 00 t^ t* 10 


1 1 + 

Tj< T}< CO (N -^ 

M 00 00 00 00 


+ 1 +• 

00 t- 
00 00 00 t^ t^ 




1 

•a-S 

It 


o o o o o 


o o o o o 


00000 


00000 


00000 




t* ?D lO '^ 00 


eg r^ O OS 00 


t^ CD UO Tf CO 


IN ^ OS 00 


t^ CD 10 Tf CO 






■<*< ■<*< T}< 00 00 


00 CO 00 CO 00 


00 CO 00 <N IN 


CM CM CM CM CM 















THE INDICATOR ON THE AIR-COMPRESSOR 59 





g 

o 

CO 

o 

8 

o 


(N lO t^ C5 

lO »0 IC IC lO 


T-i CO ■* iC »o 


CD t^ OO OS 

O OS 00 t^ r^ 

>(5 ■>*(■* Tf T)< 


CI >0 CO "5 t^ 

CO lO rf CO cq 

■* Tt< Tf rj< Tj< 


00 t^ CO 




o o o o o 


O O O o o 


O O O O O 


O O O O O 


d d d 




lO ■* CO M rH 
t- t- »^ t^ l^ 


CO lO »0 lO t* 

§22§l 


CO »0 ■<*< rf -^J* 
lO Tj< CO IN -< 

CO CO CO CO CO 


lO lO >o iC >c 


lO CO t^ 

§2S 




o o o o o 


o o o o o 


O O O o o 


O O O O O 


o o o 




lO 00 o o o 

22222 


O 00 CO »o >o 

05 00 00 00 00 


>0 rf< Tfi C<l 

■* 00 <N ^ O 
00 00 00 00 00 


1 1 ^ 

OS 00 r^ CD rti 


u,4- + 

CO IN tH 

l^ t^ t^ 




o o o o o 


o o o o o 


o o o o o 


O O O O O 


o o o 




1 lO CD 00 05 
t^ IC ■* CO (N 

o o o o o 

(N (N (N C^ IN 


05 00 »« CO (N 

^ O OS 00 t^ 

Sg222 


<-H OS t^ CO CO 
CD Tt< CO CI ^ 

2 2 2 2 2 


22222 


CO iC UO 

Tj< 00 CI 1 

OO GO 00 




o o o o o 


o o o o o 


o o o o o 


O O O O O 


o o o 




T-( O 05 00 CO 
N S^ M N ?J 


lO ■* (N 05 t^ 

|0 ^ CO ^ o 

(N (N IN C<1 <N 


CO ■* IN ^ 

g « fe 5S J5 
o o o o o 

(N IN IN C^l C^ 


1 «c^ 1 

^ cq ^ o OS 


OS OS OS 




o o o o o 


o o o o o 


o o o o o 


O O O O O 


o o O 1 




o 


Cq (N (N (N <N 


1 t^ CO 00 

(N O 05 00 CO 

c5 N ?< W C^ 


CO »0 CO 00 ^ 

W N N C^ C^ 
(N (N IN N (N 


+ 1 1 lO 

2 2 *^ 12 23 

c^ M M N cq 


CO -H o 
CI c5 M 


03 

13 


o o o o o 


O O O o o 


o o o o o 


O O O O O 


2317 
230 
2285 


ft 


00 CO IC Tj* (N 
lO lO iC ic >o 
(N <N (N (N IN 


>C CO 03 lO IN 
^ O 00 t- CO 

lO lO ■* Tf< Tf 

<N (N <N IN IN 


O OS 00 CI »c 

LO 00 CI r-H OS 
T)< -I* Tj< -^ CO 

CI IN (N (N IN 


00 ti S i -i 
CO CO CO W CO 
C^ (N IN IN CI 


3 
ft 


o o o o o 


o o o o o 


o o o o o 


O O O O O 


o o o 


§ 


+ 1 

00 tH O C5 00 
00 00 00 f^ t^ 
(N <N (N <N (N 


CO IC 00 ?3 o 
t^ t^ t^ t^ t^ 

(N C<) C^ IN (N 


C<1 00 00 W5 
OS t^ CO lO 00 
CO O CD CO CO 
(N 0< IN IN CI 


<N C^l C) O C4 


1 + 

d CI c^ 


<u 


o d> <6 d CD 


o o o o o 


o o o o o 


O o o o o 


O O O : 


03 

o 


o 


+ 1 1 

»C CO (N O 0> 
CO CO CO CO CO 


lO r-^ 00 Tt< 
CO CO CO CO CO 


00 cq CO >o 
c5 M N c5 S 


1 + 

CI O 00 t^ »c 


CO CI o ' 




o o o o o 


o o o o o 


o o o o o 


o o o o o 


o o o 




CO „ +^ 
S «c S? U5 in 

CC CO CO CO CO 


+ + 1 

00 CO lO •* (N 

•* Tj( T}< Tt< Tt< 

CO CO CO 00 CO 


+ 1 

O OS l~- »0 CO 


^ 1 + 

r^ O CC <0 ^ 

CO CO CI CI CJ 

CO CO 00 CO CO 


S?J2, 

CO CO CO 


o o o o o 


o o o o o 


o o o o o 


o o o o o 


o o o 


o 

CO 

1 


+ 1 

Tf4 Tt< Tt< ■>* ■* 


Tl< •*■*■* CO 


1 

l^ »0 00 -< OS 

CO CO CO CO CO 


4- 

t* U3 -*< W O 

00 00 00 00 00 
00 CO CO CO CO 


sfeS^ 


O O O O O 


o o o o o 


o o o o o 


_o O O O O 

1 

■* ^ OS t>" "? 
t>. t^ CO CD CD 
•^ rf< Tj< Tt< ■'t 


463 
461 
459 


lO IC Tt< T>< ■* 


+ 1 

OS 05 00 00 00 


-* CI O 00 CO 

00 00 00 r>- t^ 

Tt< Tf< ■* Tf Tf< 


o o o o o 


o o o o o 


o o o o o 


O O O O O 


616 
614 
612 


o 


.iilis 


1 

CO ■* (N O 00 

Tj( Tf Tj< Tt< CO 

CO CO CO CO «D 


CO ■* N O W 


CO •* cj o 00 

(N CI IN C< ^ 

CO CO CD CO CO 


o o o o o 


o o o o o 


o o o o o 


O O O O o 


O O O 1 


1 


+ 1 

Tf CO <N O 05 

00 00 00 00 t^ 

t* i> t^ t^ r>- 

6 6 6 6 6 


1 + 

r^ CO ■* cvi ^ 
I-. t>- t^ r* i^ 
i- t^ t^ t>- r^ 

6 6 6 6 6 


1 lO 

O 00 h- CD -^ 

t^ CO CO CO CO 

6 6 6 6 6 


+ 1 

CD CD iC iC »0 

6 6 6 6 6 


Tt< CI O 

UO »0 >0 ' 

t^ r^ t^ 

6 6 6, 


1 *^ 

r 


CO S lO •* CO 
t* N. t^ t* t^ 

d d do d 

IN ^ O 05 00 
<N (N <N ^ ^ 


1 1 + 

N ^ O OS 1^ 

t^ r^ t^ CO CO 

00 00 « 00 00 

6 6 6 6 6 


00 « « « 00 

6 6 6 6 6 


1 + 

CI ^ O 00 t* 
00 00 00 00 00 

6 6 6 6 6 


0.856 
0.855 
0.853 + 


1^ 

11 

1 


(^ CO >0 Tt< CO 


IN ^ O CS 00 

-^ -i -i d d 


rs. CO m •^ CO 
6 6 6 6 6 


CI -^ o 
odd. 



60 COMPRESSED AIR PRACTICE 

atmospheric pressure, in machines having more or less restricted inlet 
passages. 

"Tables X and XI, provide data for quickly laying out in tenths 
of a pound the theoretical isothermal and adiabatic curves on indicator 
diagrams which start their compression anywhere between 14.7 and 10 
lb. absolute. To prepare the indicator diagram for applying the tables, 
(see Fig. 9) draw horizontal pressure lines at 10-lb. intervals, to the scale 
of the indicator spring, using the portion AP of the diagram as a base 
line. Next, increase the length of the diagram by an amount equivalent 
to the percentage of the volumetric clearance in the cylinder at the end 
of the stroke, and erect the perpendicular line BC. Consider the 
length AB as one and divide it into 10 equal divisions. The tables give 
the horizontal measurements in percentages of one measured from the 
line BC; these locating the points of the compression curves on the 
various pressure lines. 

''As an example, the sketch. Fig. 9, shows a normal indicator diagram 
from an air-cylinder compressing to 100 lb., the volumetric end clearance 
being 1 1/2 per cent., with compression starting at 1 lb. below atmos- 
phere at sea-level; that is, at 13.7 lb. absolute. The diagram having 
been ruled with pressure lines and the subdivisions in length marked off, 
refer to isothermal values in Table X for 13.7 lb. absolute initial pressure. 
In the pressure columns will be found the horizontal measurements to 
be made on AB for the points in the compression curve. On the 20-lb. 
line this is 0.407, on the 40-lb. line 0.255, etc. The adiabatic values in 
Table XI for 13.7 lb. absolute initial pressure give, on the 30-lb. line 
0.439, on the 50-lb. line 0.336, etc. Thus a sufficient number of points 
are located to readily and accurately construct the curves. 

''The tables, being worked down to 10 lb. absolute pressure, may be 
used up to 10,000 ft. altitude, provided the inlet pressure does not start 
below 10 lb. 

"The tables also show the approximate position (somewhere between 
the isothermal and the adiabatic curves) of the piston in percentages 
of its stroke, for any of the given pressures, and from the isothermal 
table may be seen the relative volume of air delivered at the given pres- 
sures as compared with the original volume, considered as 1, at initial 
pressure." 

These adiabatic and isothermal curves when described are 
rather an aid to the eye in making comparisons with the actual 
compression-line of the indicator-card then necessary in compu- 
tation. The mean effective pressure of the actual card is 
determined by its area, as ascertained by planimeter or otherwise, 
and the mean effective for adiabatic and isothermal compression 
under the same conditions may be found in Table IX, and the 



THE INDICATOR ON THE AIR-COMPRESSOR 61 

economy of the actual compression may be learned by com- 
parison with them. This paragraph is only meant to apply to 
approximately sea-level computations. 



TWO-STAGE COMPRESSION CARDS 

Some one having asked in Power for ^'a diagram showing what 
constitutes a good card from a two-stage air-compressor" I 
offered a set of actual cards then recently taken from what I 
was disposed to call a good compressor, as compressors go. 




Fig. 10. — Cards from Low-pressure Cylinder. 

These cards are here reproduced exact size, Figs. 10 and 11, the 
scale of the low-pressure cards being 15 and of the high pressure 
60. 

The diameters of the air-cylinders were respectively, 32 1/4 and 
20 1/4 in., by 24 in. stroke; the diameters of the piston-rods of both 
cylinders were 3 3/8 in., and,both cylinders having the piston inlet, 
the outside diameters of the piston-inlet pipes were 10 3/8 in. and 
6 7/16 in. respectively. The net piston areas for the low-pres- 
sure cylinder are: 

Rear end (piston inlet), 732.32 sq. in. 

Forward end (piston-rod), 807.92 sq. in. 

The full cylinder capacity then per double stroke is: 

(732.32-1-807.92) X2 (feet stroke) -^ 144 = 21.39 cu. ft., 
or at 80 r.p.m., the actual running speed when the cards here 
shown were taken, 1711 cu. ft. per minute. 

The cards from the low-pressure cylinder, Fig. 10, were traced 



62 



COMPRESSED AIR PRACTICE 



as faithfully as possible, and in addition I have drawn a line in 
each approximately averaging the delivery line. The atmosphere 
line, which has disappeared in the engraving operation, was so 
close to the intake line as to be scarcely separable from it. 

The indulations which so generally occur in the delivery-line 
of air-compressor cards are a great annoyance to say the least. 
They have been attributed to the use of free-moving poppet dis- 
charge valves, and on account of this various mechanically 
moved sliding or oscillating discharge valves have been devised, 
the real motive in the designing of which has too evidently been 
not the promotion of actual efficiency but the securing of a 
smoother looking card, and yet as ragged looking discharge- 




FiG. 11. — Cards from High-pressure Cylinder. 



lines as I ever saw were on cards taken from a blowing engine 
with positively moved valves which gave a remarkably large 
and free passage for the air. 

The objections to the mechanically or positively moved dis- 
charge valve are that the precise times for it to open must vary 
continually with the delivery pressure, and also to some extent 
with the speed of the machine, so that if the valve is timed to 
open for a certain pressure it will open too late for a lower pressure 
and too early for a higher pressure, causing in each case some 
loss of power by compressing above the delivery pressure in the 
one case, and by doing compression work over again when the 
air blows back in the other. The delivery pressure in the low- 
pressure cylinder of a two-stage compressor is, of course, constant, 
being determined by the relative capacities of the high-pressure 



THE INDICATOR ON THE AIR-COMPRESSOR 63 

and the low-pressure cylinders, but this does not relieve the 
mechanically moved discharge valve as to the more difficult 
proposition: to have it give free passage for the air right up to 
the end of the stroke, and yet to have it closed absolutely before 
the beginning of the return stroke, so that all the air compressed 
and expelled will be prevented from flowing back. 

Of course it makes all the difference as to what the discharge 
valve is, its size and weight, its cushion, its spring and all that, 
which it will not do to go into in detail here. The merits of all 
the devices employed are fully set forth in the builders' catalogues. 
The promptness and efficiency of the poppet valves in closing 
in this case are sufficiently shown by the re-expansion-line, and 
the small clearance of which it gives sharp evidence. 

In computing the volume of free air taken into the low-pressure 
cylinder and actually delivered, no deduction is called for in this 
case at the beginning of the compression stroke on account of 
deficiency of pressure, as the compression-line begins fully up to 
the atmosphere line. It is to be noted that this compressor not 
only has the piston inlet, but it has the hurricane piston-inlet 
valve, a recent and decided improvement upon the original 
piston-inlet valve. As the piston-inlet valve closes by its 
own momentum at the end of the stroke, it remains 
fully open until the actual end of the stroke — the hurricane 
valve opening being 12 per cent, of the piston area — and then 
closes instantly. 

The amplitude of the opening provided by the hurricane valve 
is shown by the near coincidence of the intake line with the 
atmosphere line of the card. Although we know that the 
intake pressure in the cylinder during the stroke must be lower 
than the atmospheric pressure outside, to cause the air to flow 
in and fill the cylinder, the difference of pressure is here so slight 
that for most of the length of the stroke the intake line cannot 
be distinguished from the atmosphere line. At the precise 
moment of valve closure the momentum of the column of air 
which has been rushing through the inlet pipe is suddenly checked, 
but in the stopping of this rush there is a slight increase of pres- 
sure at the valve, which carries the pressure of the air in the 
cylinder fully up to or slightly above the atmospheric. 

This phenomenon of the final inrush of free air at the close of 
the intake stroke, and the consequent accession of pressure at 
the moment when compression is to begin, has been observed in 



64 COMPRESSED AIR PRACTICE 

compressors of several makes and with different types of air 
admission, so that the fact seems to be indisputable if still 
incredible. The observed rise of pressure, however, has never 
been enough to be worth consideration except as a curiosity. 

The actual free air delivered, as compared with the total 
capacity per stroke is as a — b:a — d. The total length a — 6 of 
the card being 3.06 in. and the length of a — d being 2.94, we have 
3.06 :2.94 : : 1.00 : 0.9607, or say 0.96, which may be called the 
actual volumetric capacity of the compressor. The free air 
actually delivered, then, per double stroke is 

21.39X0.96 = 2.53 cu. ft. 

It will be seen that the practice of deducting only the actual 
clearance space from the full cylinder content per stroke and 
making that the volumetric measure is not correct in the case 
of air compression, as the space in which this clearance air re- 
expands down to initial pressure is also to be included. The 
actual volumetric efficiency disclosed in this case is really quite 
high, although not quite as high as some apparent efficiencies 
reported. 

We now look at Fig. 10, the cards from the high-pressure 
cylinder. The diameter of the cylinder being 20 1 /4 in. , the piston 
rod 2 3/8 in. and the piston-inlet pipe 6 7/16 in., the net piston 
areas are: 

Rear end (piston inlet), 280.62 sq. in. 

Forward end (piston-rod), 313.12 sq. ft. 

The full cylinder capacity per double stroke, then, is 

(280.62 -h313.12)X2 (feet stroke) -J- 144 = 8.24 cu. ft. 

Total length of card, 3.03 in.; total air-intake length, 2.93 in. 
Percentage : 

2.93^3.03 = 0.967. 
Total intake: 

8.24X0.967 = 7.97 cu. ft. 

We found that the volume of free air taken into the low-pressure 
cylinder per double stroke was 20.53 cu. ft. This volume of 
free air at the intake pressure of the high-pressure cylinder, 23 
lb. gage, would be (temperature constant) 

23-1-14.5: 14.5:: 20.53: 7.93, 



THE INDICATOR ON THE AIR-COMPRESSOR 65 

which is remarkably close to that of the high-pressure intake 
7.97. The second cylinder at equal temperature should show 
slightly the less volume on account of probable slight leakages at 
stuffing-boxes and elsewhere. The actual result indicates that 
the intercooler brought the temperature almost, but not quite 
down to that at which the air entered the low-pressure cylinder. 
It will be noticed that I have assumed throughout 14.5 lb. as 
the atmospheric pressure, the compressor being located high 
enough above sea-level to warrant this. 



CHAPTER VI 



SINGLE-STAGE COMPRESSION 



At the present writing — although no one may say how long 
it will hold true — most of the compressed air used comes from 
the reciprocating piston type of compressor, and this it will be 
proper for us to consider first. Where a few years ago it was 
quite common practice to use single-stage machines for working 
up to 80 or 90 lb. pressure the present more general limit is 
about 50 lb. What may be said here about single-stage com- 
pression will generally apply also to the first stage of compound 
compression. 

Economy in air compression should begin at the beginning, 
and at the beginning we have to do with ''free air," or air at 
atmospheric pressure. This is our raw material and in the keep- 
ing of accounts in the air-compressing business this raw material 
is generally measured and recorded not by weight but by bulk, 
so that first of all whatever quantity of air we use it is desirable 
to get it to the compressor in as small volume as possible. The 
smaller the relative volume of air at the beginning of the series 
of operations the greater will be the profit in the end for any 
service realized. The volume of free air increases or diminishes 
as its temperature rises or falls, which means that we should get 
our free air as cold as possible. 

It seems necessary in all operations with compressed air to 
keep the accounts of profit and loss, and the record of work done, 
by the quantity of free air that is handled. This involves 
fewer uncertainties than if we were to base our computations 
upon the quantity of air after compression to any given pres- 
sure, or at any later stage in its transmission or use. 

The individual air-compressor, when the necessary deduc- 
tions have been made for clearance, for leakage, for valve action , 
for temperature procHvities, etc., when in fact its "personal 
equation" has been determined, makes a quite reliable air 
meter, and from it may be obtained a very close record of the 
free air taken in by it and compressed and delivered. After the 

86 



SINGLE-STAGE COMPRESSION 67 

beginning of operations the temperature of the air is such an 
uncertain and variable factor, and is still of such importance in 
the result, that all calculations may be upset by it. The abso- 
lute measure of the air operated upon would of course be its 
weight, but this it seems to be impossible to ascertain in exten- 
sive practical operations. While we may not deal with actual 
weight we may well bear in mind what will tend to increase or 
reduce the relative weight. 

The volume of air at common temperatures varies directly 
as the absolute temperature. With our air-supply at 60° its 
absolute temperature is 521°, and the volume of it will increase 
or decrease 1/521 for each degree of rise or fall of temperature. In 
securing our supply of free air for the compressor, then, if we 
can get a difference in our favor of 5° by laying a pipe and lead- 
ing the air in from the outside of the compressor-room, or from 
the shady side of the building, or from the coolest place near by, 
instead of using the air in the compressor-room, we accomplish 
a saving of about 1 per cent. If we secure a difference of tem- 
perature of 10°, which in practice is frequently quite possible, we 
save 2 per cent, absolutely without cost, except the first cost of 
the pipe or box to lead the air in. I know that the average 
machinist or engineer, or the man who calls himself distinctively 
the practical man, cannot commonly appreciate these small 
figures, or have any respect for such small savings, but when it 
comes to business I do not know why they should not have the 
same weight as the same values have in any other of the de- 
tails of business. Brokers have to live and flourish upon com- 
missions of 1/8 or 1/16 of 1 per cent. 

The pipe to convey the cool, free air from the point where we 
determine to take it to the compressor may as well be of wood 
or of cement or earthenware as of iron, and in fact such material 
for its non-conductivity is to be preferred. The pipe should of 
course be large enough, and with easy curves instead of sharp 
angles, so as to convey the required flow of air with perfect 
freedom. Some of the best air-compressors of the day may be 
directly connected quite readily with an outside air-supply and 
they make provision for it; others cannot easily be so connected, 
which is unfortunate for them. 

Filtering and Cooling Intake Air. — The air should not only be 
as cool as possible, but, for reasons in many cases more impera- 
tive than any power-saving conditions, it should bo free from dust. 



68 



COMPRESSED AIR PRACTICE 



In perhaps the majority of modern and up-to-date installations 
some attempt has been made to have the air clean. The arrange- 
ment adopted has usually been merely to lead in an air conduit 
from outside the compressor room and to so protect the opening 
that cats could not be sucked in, or to pipe down from the roof 
with a wide-meshed hood over the top instead of taking the air 
from the room itself. 

The means actually employed for most effectively filtering 
and cooling the intake air are generally improvised upon the 
spot and adapted to the special conditions in each case. An 
interesting and highly effective device for securing cool and pure 



* 



Revolving Sprinkler 



!^ 



Wood Framing 



':>' Burlap Covering 




Compressor 

Intake 

Pipe 



^mrr 



Fig. 13. — Air Strainer and Cooler. 



air for compressors has been brought to my notice as in use at a 
plant of the Associated Oil Company, Kernville, California, in 
the Bakersfield oil district. This device or an equivalent was 
an absolute necessity there, as the region is hot and the air is 
loaded with dust. The device is eminently common-sensible 
and in use has proved so satisfactory that it is to be recommended 
for general adoption. 

This air filter and cooler is located outside the compressor 
house, with the hooded intake air pipe standing up in the middle 
of it as shown in the sketch, Fig. 13. There is just a rectangular 
frame, or skeleton, made of light scantlings put together by a 



SINGLE-STAGE COMPRESSION 69 

carpenter. The four sides and the top of the frame are covered 
with common burlap, tacked on, and the inside is similarly cov- 
ered, with the thickness of the scantling between the two layers 
of burlap. This provides on every side two separate filtering 
surfaces for the air to pass through. Right over the center of 
the frame is located an ordinary revolving lawn sprinkler, with 
vertical axis, which slowly turns and keeps all the surfaces con- 
stantly saturated and dripping with water. The burlap in this 
case soon catches so much dust that it can be scraped off in con- 
siderable quantities, so that it is frequently necessary to souse 
the filter, inside and out, with a hose. The frame can be easily 
lifted off the intake pipe and turned over to expose the inside 
for the washing. 

This device not only effectually separates the dust from the 
air but cools it, through the evaporation of the water, and the 
air enters the cy finder in unusually good condition. The inti- 
mate contact of the water with the air and the probable absorp- 
tion of more or less water by the air has no effect upon the dry- 
ness of the air after compression. For in any case the compres- 
sion and the subsequent cooling of the air will leave it more than 
saturated, so that there will surely be free water in the air to be 
gotten rid of, after the compression, and a little more or less 
will make no difference. 

Another point which has not generally received the attention 
it deserves, although quite as important as the preceding, is 
the necessity, in the interest of the best power economy, of not 
only getting the air as cold as possible to the compressor, but 
of getting it as cold as possible into the compressor. We have 
too readily assumed that the one covers the other, when, as 
a matter of fact, it never does. The temperature of the air at 
the cylinder and about to enter it does not guarantee the tem- 
perature of the air in the cylinder at the moment when the 
cylinder is filled and compression begins. It is not too much 
to say that the temperature of the air outside the cylinder and 
of that inside is never the same. Yet it is not to be forgotten 
that the sole object of the effort to get cool air for the com- 
pressor is to have it as cool as possible, and of as small a vol- 
ume as possible, at the moment when compression begins. How 
cool the air may have been at any previous moment, however 
near, has nothing to do with the case. 

In another chapter I have remarked that the air-com- 



70 COMPRESSED AIR PRACTICE 

pressor is the ideal and the only perfect field for the use of the 
indicator, that it is the only place where the indicator diagrams 
will tell the whole story both of the power expended and of the 
work accomplished. This is undoubtedly true, but it is a 
statement that is quite likely to be understood to say more 
than it does say. The indicator diagram from the air-cylinder 
does not tell all that it seems to tell, or it sometimes tells it 
wrong. You may note upon the diagram the point, very near 
the beginning of the compression-stroke, where the cylinder, 
if we may believe the diagram, is filled with free air, or air at 
atmospheric pressure, and from that, after deducting what 
fills the clearance-space at the end of the stroke, we may compute 
the volume of free air actually compressed and delivered; and 
then, later, we may realize that we have not got the volume of 
free air that the diagram testifies to. This is due to the fact 
that the diagram has nothing to say about the actual tempera- 
ture of the air, either at its admission, at its discharge, or at any 
point of the stroke. 

With steam, unless it is superheated, the pressure indicated 
guarantees the temperature; with air the pressure and the 
temperature have no necessary connection. I might show you 
a diagram from an air-compressing cylinder where the air- 
admission line is almost exactly coincident with the atmosphere 
line, and where the compression line begins to rise above the 
atmosphere line immediately at the beginning of the compres- 
sion-stroke, showing that the cylinder is completely filled with 
air at atmospheric pressure, and we may congratulate ourselves 
that the diagram is an excellent one in this respect; but suppose 
that when the cylinder is just filled, and compression is just 
beginning, our cylinder is filled with air at 120° instead of at 
60°, which is the temperature of the supply. It means that 
our cylinder holds rather less than 0.9 of the air that we are 
assuming that it holds, and which the diagram says that it 
holds. 

It means not merely that the practical capacity of the com- 
pressor is one-tenth less than we assume it to be, but that for 
the compression of this nine-tenths we are still expending the 
full power as represented by the steam-card. If the difference 
in indicated horse-power between the air-cylinder and the steam- 
cylinder is 10 per cent, of the air-cylinder, or if the power ratio of 
the steam to the air be 1.1:1, it is not a bad showing. But 



SINGLE-STAGE COMPRESSION 



71 



if this 1.1, the power of the steam-cylinder, is to be compared 
not with 1, the full capacity of the air-cylinder, but with .9, its 
actual contents, the case is quite different: 0.9:1.1: :1: 1.22, 
which is a result not worth bragging about by any compres- 
sor builder. 

There seems to be no means of ascertaining the actual tem- 
perature of the air during the operation of compression. The 
temperature of the air at different points of the stroke would 
be easily computable from the indicator diagram, which shows 
the pressure attained at any point, if we only knew the initial 
temperature, but as we have no means of knowing the initial 
temperature we do not know the actual temperature at any 
time. Who will tell us how to find it out? This does not 
seem to be an impossible problem. It looks at first sight al- 
most as simple — not quite — as to tell how fast a stream of 




Section on A-B Section on C-D Section on E-F 

Fig. 14. — Novel Air Intake. 



water flows through a pipe. But nobody has yet invented a 
perfect water-meter. In the meantime we can only use our 
mechanical judgment and common sense as to the best means 
of getting the air into the cylinder as cool as possible. We 
can say in a general way that the air should enter the cylinder 
by the shortest and most direct possible passage, and with as 
little contact as possible with any metal at a higher temperature 
than its own. 

All this suggests and invites investigation and discussion as 
to the various means adopted by the different designers for 
getting the free air into the cylinder, the directness or tortuosity 
of the passages and the temperature of the surfaces which the 
air must pass, but this is not within the scope of the present 
publication. Some arrangements are undoubtedly better than 



72 COMPRESSED AIR PRACTICE 

others, and the survival of the fittest may be expected to work 
itself out here as elsewhere. 

Fig. 14 is a sketch of the air-cylinder of a compressor of Eng- 
lish manufacture which is presented here for whatever it may be 
worth. It appeals to the writer as likely to get the air into the 
cylinder somewhat cooler than is done by some other and more 
familiar arrangements, although some other features of the 
design may not be so admirable. 

It will be seen that the cyhnder is abnormally long for the 
piston stroke, and that the piston faces are widely separated, 
or in fact comprise two pistons fixed upon the same rod. A 
single air intake passage for both strokes surrounds the middle of 
the cylinder with numerous radial openings direct into the 
cylinder. The face of each piston carries a large annular valve 
of slight movement, covering liberal openings through the piston. 
This valve is quite heavy and is provided with a regular packing 
ring which fits the periphery of the cylinder, causing friction 
which operates the valves, thus opening or closing the piston 
passages upon each reversal of the stroke. This arrangment 
permits the most complete water-jacketing. 

Returning now to the more familiar, or what we may call the 
standard type of single- cylinder compressor, and coming to the 
completion of the stroke when the air compressed has been 
expelled or delivered, the air remaining in the clearance-space 
between the piston and the cylinder head, both considerably 
heated by previous charges of air, although it may not have lost 
much of its heat of compression it must have lost some on 
account of the cooling influence of the water jacket, and its 
temperature cannot be quite as high as the theoretical 
temperature due to the compression. Still it is compara- 
tively hot, and when it is remembered that this hot air 
becomes a part of the next cyhnderful of air to be com- 
pressed it has been assumed that therefore the mean tempera- 
ture of the contents of the cylinder is somewhat increased by 
this admixture. But this conclusion is hasty and unwarranted. 
This hot air in the clearance-space is only hot when under 
the terminal pressure, and as at this pressure it is not as hot 
as the theoretical temperature for the given compression it can- 
not upon its re-expansion to atmospheric pressure be as hot as 
it was before its previous compression began. It must be really 
somewhat cooler than the air that rushes in to fill the cylinder 



SINGLE-STAGE COMPRESSION 73 

for the next stroke, and it therefore does not contribute any heat 
to the new charge of air, but rather receives some heat from it 
and thereby sHghtly cools it. 

The face of the piston is exposed to the compression-heated air 
only until the re-expansion of the air in the clearance-space has 
occurred, and thus for almost the entire return stroke it is ex- 
posed to cooling rather than to heating influences. 

The air remaining uncompressed in the clearance-space at the 
end of the compression-stroke, as it does not raise the, temperature 
of the incoming air or tend to increase its volume, has therefore 
no bad effect in that respect, and in no way increases the" power 
required for compressing a given quantity of air. The power 
that has been expended in the compression of this air in the clear- 
ance-space is not lost, or but a portion of it, as it gives out in 
its re-expansion, by helping the piston upon its return stroke, 
most of the power expended in its compression. Clearance in 
the air-cylinder, therefore, represents a loss of capacity in the 
air-compressor rather than a loss of power. And it is chiefly on 
account of its reducing the capacity of the compressor to com- 
press its full quota of free air per stroke that it is desirable to 
keep the clearance as small as possible. 

Leaky Discharge Valves. — In attempts to explain the causes of 
the ignitions and explosions which sometimes occur in air 
receivers and pipes, it is frequently suggested that the high air 
temperatures which are assumed to precede these occurrences are 
caused by leaky discharge valves, which allow a certain portion 
of the compressed and heated air to return into the cylinder 
retaining its high temperature there to be re-compressed and 
heated still more, this operation being repeated over and over 
until the air becomes so hot that if there is anything combustible 
or explosible in the neighborhood this red-hot air will be ready to 
touch it off. This ''theory," variously worded, has been enun- 
ciated and repeated in technical journals of high standing with 
little protest from any source. No one has volunteered an ex- 
planation of how any portion of a cylinderful of air could be 
isolated from the rest and be churned back and forth in this 
way instead of mixing with and being carried along in its regu- 
lar place in the procession, or how the air could continually 
re-expand as it leaked back and still retain its full high 
temperature. 

Leaky discharge valves have still another scapegoat job. It 



74 COMPRESSED AIR PRACTICE 

seems to be a fact — at least indicator-cards say so — that in 
certain types of compressors — and there are more than one of 
them in which it occurs — if the intake air be led to the com- 
pressor through long unobstructed passages of not too great 
area, and if the inlet valves provide a free opening into the 
cylinder right up to the moment when compression begins, the 
cylinder may be filled with air quite up to and even a perceptible 
amount above atmospheric pressure, this high and untheoretical 
pressure being apparently caused by the sudden checking of the 
momentum of the rapidly moving column of air. 

There are those who, while they cannot dispute the evidence of 
the indicator, refuse to accept the explanation. They say that 
any intake pressure quite up to or slightly above atmospheric 
pressure is not legitimately possible, and that when the indicator 
says that such pressure exists it can only be properly explained 
as caused by leaky discharge valves. The absurdity of this 
proffered explanation would seem to be self-evident. 

The explanation is offered by respected professors and others 
in the presence of an entire indicator-card not otherwise abnormal. 
At the beginning of the return stroke the re-expaneion-line drops 
promptly to slightly below atmosphere, showing that the dis- 
charge valves do not leak. Through the entire intake stroke the 
air line runs closely parallel to and shghtly below the atmosphere 
line, showing that the discharge valves do not leak, but at the 
moment when there is the entire cylinderfull of air whose pres- 
sure theoretically should be slightly below atmosphere then there 
is such an inrush from the leaky discharge valves as to almost 
instantaneously raise- the pressure of the entire cylinderful of 
air a half a pound or a full pound. When the compression stroke 
begins the compression-line is perfectly normal again, which it 
could not be if the discharge valves were still copiously leaking 
back. Obviously the sudden checking of the inrush of the air 
by way of the inlet pipe offers the more acceptable explanation. 

Supposing that we are filling the air-cylinder by the natural 
inflow of the air under the pressure of the surrounding atmos- 
phere, and that we have got into the cylinder the greatest pos- 
sible actual weight or quantity of air under those conditions, 
and, assuming that the air is also as cool as we can get it, we may 
then be said to have got our material as cheaply as possible, to 
have started our business under the most favorable conditions, 
and with encouraging prospects; and we may then, and not 



SINGLE-STAGE COMPRESSION 75 

until then, consistently and without reproach look for the avail- 
able means of economy in the actual operation of compression. 
The same considerations that tend to economy in the procuring 
of the air, or of getting it into the cylinder, hold good also in all 
the subsequent operations of compression. The smaller the 
bulk or volume of any given quantity or weight of air the cheaper 
can the compression be effected and the better will be the econ- 
omy; and, as the volume of the air at any given pressure depends 
upon its temperature, the supreme consideration throughout the 
operation is to keep the air as cool as possible. Keeping the air 
cool during compression means actually cooling the air during 
compression. No compression can be effected without a corre- 
sponding rise of temperature in the air compressed. Theoretic- 
ally the rise will always be the same where the condit'ons are 
identical. Starting with a given volume of air and with the air 
at a given pressure and temperature, and compressing to another 
and higher pressure, the resulting volume and temperature should 
always be the same. Practically the temperature of the air 
after compression, or during compression, is never as high as the 
theoretical temperature, or as high as the books and tables say 
that it should be, and it is also widely variable under apparently 
slight changes of conditions. 

This is not at all because the theory in the case is incorrect, 
but rather that it is incomplete, in that it is not cognizant of all 
the conditions that affect the case. Theory says, and correctly, 
that the element of time has nothing to do with the heat of 
compression; that a given volume of air when compressed to 
another given volume will have its temperature raised so much, 
whether it takes a minute, an hour, or a week to do it. Practi- 
cally time has a great deal to do with the case. The readiness 
with which the air will receive heat from or impart it to what- 
ever may be in contact with it, and the small amount of heat 
actually represented by its changes of temperature render the 
actual volume a highly elusive quantity, and time becomes a 
playground for it. 

In a compressing-cylinder in actual use all the parts of it, the 
body of the cylinder, the heads, the piston and rod, the valves 
and seats or guides become heated by their contact with the 
compressed air; but while they are thus becoming heated they are 
only heated by this contact, and while being heated they are also 
being cooled, as they are constantly transmitting some of the 



76 



COMPRESSED AIR PRACTICE 



heat received from the air and dispersing it by conduction or 
radiation; and, consequently, these parts are never as hot as 
the air that heats them — when the air is at its hottest — and the 
air also is not as hot as it would have been but for its contact with 
them. The metallic parts after a time of continuous operation 
attain an average temperature, and will not get any hotter. The 
mean temperature attained will depend upon the facilities pro- 
vided for taking the heat away. Nothing better is known or 
has been suggested for conveying away the heat than cold water. 
When the entire compression is effected in a single cylinder 
the heat of compression is abstracted from the air mostly at the 
latter part of each stroke, when the air is at its hottest and when 
the difference in temperature between the air audits surroundings 




Fig. 15. — When Compression Line Leaves the Adiabatic. 



is the greatest. Indeed it is to be supposed that in active com- 
pression the air loses none of its heat of compression during the 
earlier part of the stroke unless the means of cooling the cylinder 
parts are unusually efficient and operative. If at the beginning 
of the stroke the cylinder is hotter than the air, as it naturally 
must be, the air is naturally heated rather than cooled by the 
contact. Practical evidence of this is not wanting. Indicator 
diagrams from air-compressing cylinders are easily to be found, 
as Fig. 15, where the compression-line of the diagram does not 
leave the adiabatic line until the first quarter of the stroke is 
traversed. In this connection it may be remarked that for evi- 
dence upon the point that we are considering any indicator- 
cards that are taken when a compressor has just been started. 



I 



SINGLE-STAGE COMPRESSION 77 

and before the cylinder parts have attained their full average 
temperature, are not to be considered. Such cards promise 
better than the actual performance of the compressor will fulfil. 
The heating of the air does not continue throughout the 
whole stroke of the piston, but is accomplished and ceases at 
the moment that the full pressure is reached; and for the remain- 
der of the stroke, while the compressed air is being ejected from 
the cylinder, the air is becoming somewhat cooler, while the metal 
inclosing it is becoming hotter. The heat of the cylinder parts 
is not evenly distributed. The ends of the cylinder and the entire 
cylinder-heads, being exposed to the air when it is hottest, 
naturally become hotter than the middle of the cylinder, which 
never feels the hottest air. The importance of the water-jacket, 
in the absence of any better cooling device, is obvious enough. 
The cooling effect of the water is greater when it is applied to 
the cylinder-heads than anywhere else, because they are exposed 
to the heated air for the greater portion of the stroke, while the 
inner surface of the cylinder itself is covered by the advancing 
piston. Apart from the cooling of the air under compression, 
and the reduction of its volume, the water-jacket is a necessity 
as affecting the lubrication of the cylinder surfaces. Without 
some such means of cooling the cylinder it would become so 
heated as to burn the oil and render it useless as a lubricant. 

The device of cooling the air by the injection of a spray of 
water into the cylinder is probably the most effective cooling 
arrangement that has ever been devised, but collateral objec- 
tions have driven it completely out of use in all new compressors 
at least, in the United States. When the spray is used the 
success of it as an air-cooling agent is entirely dependent upon 
the mode of its application. The spray can only possibly 
effect the intended purpose when diffused through the air while 
, it is being compressed, or during the compression-stroke of 
the piston. It can only cool the air while it is hot, or while it 
is being heated; so that to admit the water with the incoming 
air is only to let it fall inert and useless to the bottom of the 
cylinder, to be driven out by the piston. Air so admitted may 
have a quasi usefulness in filling the clearance-space af the end 
of the stroke, but it can do little or nothing toward cooling 
the air. The presence of the water may also make it unsafe 
to run the compressor at a speed that would be otherwise safe 
and proper. With the use of water in the compression cylinder, 



78 



COMPRESSED AIR PRACTICE 



whether properly injected or not, no satisfactory means of 
lubricating the surface of the cylinder has ever been found, 
so that the friction of the piston and the loss of power by that 
means is greater than with other systems of compression. The 
piston and cylinder surfaces also wear away rapidly, so that 
the repair costs and the inconveniences entailed are greater than 
with other systems. 

Other things being equal, a cylinder of small diameter has a 
decided advantage over a large one in cooling the air during 
compression. In a large cylinder the portion of air immediately 




[ Fig. 16. — An Actual and Nearly Perfect Indicator Card. 



in contact with or lying near to its water-cooled surfaces will 
be cooled by the contact, but the air in the middle of the cylinder 
will be little and slowly affected. A number of small compres- 
sors will show better results, as regards the cooling of the air, 
than a large compressor can show. 

The indicator-card here reproduced. Fig. 16, which came into 
my possession more than a score of years ago, is still the best 
and most satisfactory diagram from a single-stage compressor 
which I have ever seen. It was taken from one of a series of 
small compression- cylinders entirely submerged in a tank of 
water. The scale of the diagram is 30. 



CHAPTER VII 
TWO-STAGE AIR COMPRESSION 

In the preceding chapter only single-stage compression was 
considered. When air is to be compressed to pressures above 
40 or 50 lb. and up to 200 lb. or so, two-stage compression 
should be employed, and within the range suggested the higher 
the pressure the greater the need of it. The two-stage plan is 
adopted for the sake of the cooling of the air and the consequent 
reduction of volume before the second compression, so the inter- 
cooling is the essential detail of the operation, and without that 
two-stage compression could be of no advantage. 

Not only is there generally an appreciable saving of power in 
two-stage compression, but, perhaps more important, is the 
avoidance of the high temperatures, thus permitting more 
satisfactory lubrication, greatly reducing the deposition of com- 
bustible material in air receiver and pipes and minimizing the 
liability of fires and explosions. Single-stage compression seems 
to be responsible for most of the explosions which occur, as 
will appear elsewhere. 

The first point to be considered in two-stage compression 
is the equitable distribution of the load between the two cylinders, 
which would be determined by their relative capacities, and 
these required relative capacities would vary with the ratio of 
the initial and terminal air-pressures. If the pistons of both 
the air-cylinders have the same stroke then their volumetric 
capacities will be as the squares of the cylinder diameters, and the 
ratios of these should be as the square root of the number of com- 
pressions required. 

Thus in working at sea-level and compressing and delivering 
air at 90 lb., the ratio of pressures would be (90+14.7)^14.7 
= 6 . 44 and \/q . 44 = 2 . 53 . Then say that our intake or low-pres- 
sure cylinder was 20 in. in diameter, to find the diameter of the 
high-pressure cylinder: 20^ = 400, 400^2.53 = 158 and ^158 = 
12.56 for the diameter of the high pressure, or sayl2 1/2 in. 

79 



80 COMPRESSED AIR PRACTICE 

The concise formula for the abve operation is: 



Ut 



d= I /P.-fp 



7)2 

2 



Here D is the diameter of the low-pressure cylinder 
d is the diameter of the high-pressure cylinder 
P] is the initial absolute air-pressure 
P2 in the terminal absolute air-pressure 

It is evident that the ratios of cylinder capacities must vary 
with the fixed working conditions, and a compressor properly 
adapted for sea-level work and for comparatively low delivery 
pressure would be far from its best if working at a high altitude 
and high delivery pressure. 

Computing by the above formula, if we had a low-pressure 
cylinder 30 in. in diameter working at sea-level to compress to 
80 lb. the diameter of the high-pressure cylinder would be 18.84 
in.; while to compress to 120 lb. the diameter would be 17.26 
in. Working at an altitude of 10,000 ft. (normal air-pressure 
10 lb.) and compressing to 120 lb. the high-pressure cylinder 
diameter would be 15.81 in. In building compressors it is 
not customary to work to any computed cylinder diameters 
nearer than 1/4 in., and quite generally the nearest whole inch is 
the figure adopted. 

Three -stage Compression. Ratios. — For three-stage com- 
pression, which is advisable for pressures up to 1000 lb., the cube 
root instead of the square root of the ratio of initial and terminal 
air-pressures becomes the ratio of the successive cyhnder capac- 
ities, the formula then being: 



I D^ 

d= /3/P1+P2 

Here D is the diameter of the first cylinder when computing 
that of the second, but it is also that of the second cyhnder when 
computing the diameter of the third, and d is the diameter of 
either the second or the third cylinder, as computed from that 
of the cylinder which precedes it. 

Say that we have a cyhnder 30 in. in diameter as before, this 



TWO-STAGE AIR COMPRESSION 81 

diameter being fixed upon for the free-air capacity of the machine, 
to compress to 1000 lb. the ratio of initial and terminal absolute 
pressures will be 70, and V70 = 4.12 which will be the ratio of 
cylinder areas. Then 302^4.12 = 218 and V21S = U.7Q di- 
ameter of second cylinder. The area of this cylinder being 218, 
then we have 218^412 = 53, and \/53 = 7.28, diameter of third 
cylinder. 

Four-stage Compression. Ratios. — For four-stage compres- 
sion the fourth root of the ratio of initial and terminal absolute 
pressure is taken as the ratio of the successive cylinder capacities, 
the formula then becoming: 



m^ 



Here as before D is the diameter of the largest cyUnder and 
also of the two following cylinders as compared with the next 
smaller one and d is the diameter of each smaller cylinder as 
compared with its immediate predecessor. 

With an intake cylinder 20 in. in diameter what will be the 
successive diameters of the other cylinders to compress by four 
stages to 2500 1b.? 

2514.5-7-14.5 = 173; the ratio of absolute pressures. 

■\/l73 = 3.62 ratio of successive cylinder capacities. 
400 -f-3. 62 = 110 \/llO =10.48 diameter of second cylinder. 
110 ^3.62 = 30.4 VSO . 4 = 5.5 diameter of third cylinder. 
30.4-^3.62=8.4 \/ 8.4= 2.9 diameter of fourth cylinder. 

If, as often is the case, the last cylinder was single acting, 
then its diameter would be V 8.4X2 = 4.1 

The Distribution of the Load. — In stage compression the 
proper determining of the relative cylinder capacities is only a 
part of the problem ; the location or arrangement of the cylinders 
to secure an equitable distribution of the load is also of importance- 
It happened that nearly at the beginning of my apprentice, 
ship to compressed air I was brought into rather intimate con- 
tact with a two-stage compressor from which many things were 
to be learned — some of them to be unlearned later. 

This was a so-called straight-line machine with single-acting 
tandem air-cylinders, the two air pistons upon the same rod with 

7 



82 



COMPRESSED AIR PRACTICE 



the steam piston, and the alternate work of the two air-cyUnders 
being done upon the alternate strokes of the steam-engine. The air- 
cylinders were 20 in. and 11 3/4 in. respectively and the stroke 18 
in. The capacity ratio of the two cylinders, deducting the area 
of the piston rod in the larger cylinder was 1:0.35. 

Indicator-cards were taken from both air-cylinders with the 
compressor delivering air at 35 lb., at 40 lb., and then by inter- 
vals of 10 lb. all the way up to 120 lb. The cards here presented 




Fig. 17. 




Fig. 18. 




Fig. 19. 
Figs. 17-19. — Indicator Cards from Two-Stage Single Acting Tandem 

Cylinders, 

are as good as a greater number for bringing out the peculiarities 
of the case. Fig. 14 is from the first or low-pressure cylinder. 
This card did not vary in any particular throughout the whole 
series from 35 lb. to 120 lb., and it would have continued the 
same no matter how high the terminal or deUvery pressure of 
the second cylinder were carried. A tracing was made of one 
of these cards and laid over several others of the series, and the 
variation was so slight as to be scarcely discoverable at any 
point. 

The mean effective pressure of Fig. 17 is 15.8 lb., and the 
terminal pressure is 35 lb. While the terminal pressure in this 



TWO-STAGE AIR COMPRESSION 83 

first cylinder is 35 lb., it does not mean that if the two-stage 
compressor were compressing and delivering air at 35 lb. gage, 
the first cylinder would be doing ail the work of the compressor. 
It is to be remembered that the complete work of air compres- 
sion comprises two distinct operations: the compression of the 
air to the 'required pressure, and the expulsion or delivery of 
the air against practically the same pressure in the air-pipes or in 
the air-receiver. In the case that we are now considering, 
where the air is delivered from the compressor at a pressure of 
35 lb., the first cylinder happens to do all of the work of compres- 
sion, and none of the work of expulsion or delivery. In any case 
of two-stage compression, if either cylinder is to be called dis- 
tinctly the ''compressing" cylinder, that term always belongs to 
the first cylinder rather than to the second. If our two-stage 
compressor were delivering air at a pressure higher than 35 lb., 
the first cylinder would still compress the air to 35 lb. as before, or 
would do only a portion of the total compression, and of course 
none of the delivery. The height to which the first cylinder 
will continually compress the air is determined by the relative 
capacities of the two cyHnders, modified to some extent by the 
cooling of the air that may be effected in its passage from one 
cylinder to the other. The work of the second cylinder when 
the compressor is delivering the air at 35 lb. is shown by Fig. 18, 
taken from that cylinder. The delivery-line ha in this case 
would be a perfectly horizontal line if the movement of the piston 
were uniform throughout the stroke, the rise and fall of the line 
corresponding approximately to the acceleration and retardation 
of the piston. 

At whatever pressure the compressed air may be delivered by 
the compressor the mean effective pressures for the two distinct 
operations of compression are never alike. The mean effective 
pressure for compression only is always lower than the M.E.P. 
for delivery only, and of course also lower than for the combined 
operation of compression and delivery as performed in a single 
cylinder. In the compression Table IX, columns 6 and 7 give 
the mean effective pressures for the whole stroke when all of the 
work of compression and delivery is done in a single cylinder, 
column 6 being for isothermal and column 7 being for adiabatic 
compression. In the same table columns 8 and 9 give respec- 
tively the isothermal and the adiabatic M.E.P. for the compression 
part only of the stroke of a single air-cylinder. 



84 



COMPRESSED AIR PRACTICE 



Resuming now our compound compression, and referring 
again to Fig. 17, we notice that its mean effective pressure — 
15.8 — is greater than the pressures given in either columns 
8 or 9 for compression only to 35 lb., where the entire work of 
the compressor is done in a single air-cylinder. The table 
referred to, as we have previously stated, has nothing to do with 
compound compression, but the comparison of figures might 
provoke suspicion that in compound or two-stage compression 
we are doing the same work of compression as in the single 
air-cylinder, but at greater expense, and it is therefore proper 
to refer to it here. 

The case represented is different in more than one particular. 
In single-stage compression the compression is all done in the 
one cylinder, and throughout the entire compression-stroke the 
same quantity of air is acted upon. In Fig. 17 we are not doing 
the entire compression part of the work in the one cylinder, 
although it is begun there, and the weight of air acted upon is 
not the same throughout the stroke. While at the beginning 
of the stroke the air acted upon is the free air contained in the 
first cylinder and just admitted from the atmosphere, this 
continues only for the first half of the stroke, and for the latter 
part of the stroke the whole body of air then undergoing com- 
pression consists not only of all the contents of the first cylinder 
that have not been expelled by the advancing piston, but also 
of the entire contents of the passage connecting the two cylinders, 
and the contents of that part of the second cylinder which has 
been vacated by its retreating piston. Fig. 17 shows the com- 
pression beginning at a, at the beginning of the stroke, and 
with the free air contents of the first cylinder alone. 

This goes on until the point o is reached, near the middle of 
the stroke, and then communication is opened with the air- 
passage that connects the cylinders, and through that with the 
second cylinder. When the previous compression-stroke of the 
first cylinder ended, the passage connecting the cylinders was 
filled with air compressed to 35 lb., and by the action of the valves 
this passage was then for a time shut off from communication 
with either cylinder. This passage, in fact, remains shut off 
from communication with either cylinder during the whole of 
the return stroke, while the first cylinder is being filled with a 
fresh charge of free air, and while the compressed air in the smaller 
cylinder is being expelled into the discharge-pipe and the air- 



TWO-STAGE AIR COMPRESSION 85 

receiver. When the return or intake stroke of the larger cylinder 
has ended, which return stroke is the delivery-stroke of the smaller 
cylinder, and when the compressed air has all been expelled 
from the smaller cylinder by its piston reaching the end of it, then 
the return stroke of the smaller cylinder commences, this stroke 
being of course coincident with the next compression-stroke of 
the larger cylinder. 

With the commencement of the return stroke of the smaller 
piston the air confined in the connecting passage begins to re- 
expand and to flow into the smaller cylinder. The pressure is 
thus falling in the air-passage, on account of its supplying the 
smaller cylinder, and at the same time compression is going on 
in the larger cylinder, and the pressure in it is rising. These 
simultaneous operations go on until at length the point o is 
reached, where the pressure in the larger cylinder exceeds the 
pressure in the air-passage and in the smaller cylinder, and the 
air from the larger cylinder begins to flow into the air-passage, 
and at the same time the entire contents of the air-passage and 
of the smaller cylinder become constituent parts of the body of 
air that is being compressed by the advancing piston of the larger 
cylinder, and thereafter until the end of the stroke the com- 
pression of the combined contents of large cylinder, air-passage, 
and small cylinder goes on together. The last one-third of the 
compression-stroke in Fig. 17 and the portion ub in Figs. 18 or 
19 represent the same operation of compression, the line in Fig. 
17 showing a somewhat higher pressure than in Fig. 18 or 19 on 
account of the friction to be overcome in passing the valves and 
passages. 

The mean effective pressure for the combined operation of 
compressing and expelling the air at 35 lb., or for the whole 
operation of air compression so termed, when performed adiabat- 
ically in a single cylinder is, theoretically, 21.6 lb. Practically, 
without any special arrangements for cooUng the air, the M.E.P. 
usually falls somewhat below the above figure, as the air inevit- 
ably loses more or less of its heat during the operation. If 
we consider Fig. 17 in connection with Fig. 18, they together 
represent the whole operation of compression to 35 lb. by two- 
stage compression. Fig. 17 representing the compression of 
the air and Fig. 18 representing its expulsion or delivery. The 
mean effective pressure of Fig. 17 is, as we have seen, 15.8, 
and that of Fig. 18 is 16.4 lb. But it must be remembered 



86 COMPRESSED AIR PRACTICE 

that the diaireters of the two cyhnders are quite different, 
and 16.4 lbs. in the 11 3/4 in. cyhnder is only equal in power to 
5.65 lb. in the 20 in. cylinder, and 15.8+5.65 = 21.45 lb., a 
mean effective pressure quite close to what might have been 
expected for the entire operation of compressing air to 35 lb. 
without any device for cooling the air. When we remember 
that the use of two cylinders instead of one for the same operation 
of compression means necessarily a greater first cost for the 
apparatus, to the builder if not to the purchaser, a larger number 
of parts, increasing the liability to accidents and delays, and a 
greater amount of friction, both in the air and in the machine, 
to be constantly overcome, it is evident that two-stage com- 
pression of itself costs more than single-stage compression. 

While these diagrams were being taken the compressor 
was run at about 80 r.p.m., or 240 ft. of piston travel per minute, 
throughout. At this speed the indicated horse-power of 
Fig. 17 for the first cylinder is 18 . 05 and that of Fig. 18 from the 
second cylinder is 6.46, their sum being 24.51. Fig. 19 is from 
the smaller cyhnder when compressing to 70 lb. The M.E.P. 
of Fig. 19 being 43.4, and the indicated horse-power being 
17.1, the i.h.p. for Fig. 17 being, as before, 18.05, their sum 
is 35.15. When compressing and delivering air at 70 lb., as 
indicated by Figs. 17 and 19, it will be noticed that the i.h.p. of 
the two cylinders is nearly equal, and it would thus seem that 
the ratio of the cylinder capacities to each other was approxi- 
mately correct for that pressure. The relative diameters 
and areas of the two cylinders may have been determined 
upon this assumption, which involved an ignoring of essential 
particulars. 

The arrangement of the tandem, single-acting, two-stage 
compressing cyhnders is about as bad a one as could be devised 
for an air-compressor, and no possible change in the relative 
capacities of the two cylinders can make it right. The trouble 
in the case is that while the sum of the indicated horse-powers 
as computed from the actual enclosed areas of the two cards 
is correct as representing the total horse-power consumed in the 
operation, it does not correctly represent the actual distribution 
of the resistances as encountered in the opposite strokes of the 
engine. The back pressure in the second cylinder, which thus 
far has not been thought of, imperatively demands recognition 
and accounting with, as modifying the total resistances encoun- 



TWO-STAGE AIR COMPRESSION 87 

tered. The back-pressure line, or, perhaps more correctly, 
the return-pressure Hne, cxuh, as we have seen, starting at 
c, represents for nearly one-half the stroke the. re-expansion 
of the contents of the air-passage. This re-expansion goes 
on in the passage and in the smaller cylinder combined until 
the point x is reached, when the compression going on in the 
larger cyUnder has brought its contents up to the same pres- 
sure. Then after a short interval, xu, occupied in securing a 
sufficient excess of pressure, and in reversing the movement 
from expansion to compression, the compression continues 
from u to the end of the stroke, when the pressure of 35 lb. is 
again reached. 

As the whole of Fig. 17 is always the same, no matter what 
may be the working pressure of the compressor, so that it is 
not below 35 lb., so also the return line of the diagram from the 
second cylinder is always the same, and the only change in the 
pair of Figs. 17 and 18 or of 17 and 19 for different delivery-pres- 
sures is in the upper line ha, the compression- and delivery-line 
of the second cyhnder. When compressing to 35 lb. only, there 
is no compression in the second cyhnder, and its whole stroke 
is occupied in delivery. At the beginning of the stroke the 
resistance against the high-pressure piston is represented by 
the height of the vertical line bd. The resistance at any point 
of the stroke would be represented by a vertical line at that 
point drawn from the line ha down to the atmosphere-line, and 
the total resistance for the working-stroke is represented by the 
enclosed area, hdea. This means that the total back pressure, 
hdec, is to be added to, or, rather, is not to be deducted from, 
the work of the compression- and delivery-stroke of the high- 
pressure cyhnder. During this working-stroke of the high- 
pressure cylinder the low-pressure piston is making its return 
stroke and allowing its cylinder to refill with air at atmospheric 
pressure. 

The pressure upon each side of the low-pressure piston upon 
its return stroke is practically that of the atmosphere, and 
therefore no resistance of any magnitude is to be taken into ac- 
count as increasing or diminishing the total work of the high- 
pressure cylinder for its delivery-stroke. When, however, the 
low-pressure cylinder is doing its work of compression, it is 
assisted in its work by the return or back pressure of the high- 
pressure cylinder, which acts upon the high-pressure piston in 



88 



COMPRESSED AIR PRACTICE 



the same lineai direction as the low-pressure piston is travelling. 
The back pressure, hdec, which is added to the work of the 
high-pressure cylinder for its delivery-stroke, as represented 
by the enclosed area bac, is to be deducted from the work of 
the low-pressure cylinder for its compression-stroke as repre- 
sented by Fig. 17. 

If now we go over the series of indicator-cards, computing 
the indicated horse-power of each, adding the i.h.p. of the 
back pressure to the i.h.p. of each of the high-pressure cards, 
and deducting the same from the i.h.p. of the low-pressure card, 
as above described, we find that the net resistance for the 
alternate strokes is very inequitably distributed. The figures 
for compressing to 120 lb. are also given to aid the comparison 
although the delivery or high-pressure card for that pressure 
is not shown. The case will stand like this: 

M.E.P. of low-pressure cylinder 15.8 lb., i.h.p. 18.05. 

M.E.P. of return stroke of high-pressure cylinder 20.1, 
i.h.p. 7.88. 

Then 18.05-7.88 = 10.17, the constant net i.h.p. for the 
compression-stroke of the low-pressure cylinder or the return 
stroke of the high-pressure cylinder. 

M.E.P. of high-pressure cyl. at 35 lb. 16.4, i.h.p. 6.46. 

M.E.P. of high-pressure cyl. at 70 lb. 43.4 i.h.p. 17. 1 

M.E.P. of high-pressure cyl. at 120 lb. 65.7 i.h.p. 25.89. 

Then adding to these results the i.h.p. for the return stroke, 
which should not have been deducted from the delivery-stroke, 
we have: 

6.46-1-7.88=14.34 when delivering at 35 lb. 

17.1+7.88 = 24.98 when delivering at 70 lb. 

25 . 89 + 7 . 88 = 33 . 77 when dehvering at 120 lb. 



As these several results for the delivery-stroke are successively 
to be compared with the constant i.h.p. 10. 17 for the initial com- 
pression-stroke, it will be seen that even when delivering the air at 
but 35 lb. the delivery-stroke of the high-pressure cylinder takes 
nearly 11/2 times the power required for the return stroke. When 
compressing to 70 lb. under the above arrangement the delivery- 
stroke takes nearly 2 1/2 times the power of the return stroke, 
and when compressing to 120 lb. it takes more than 3 times as 
much. 



TWO-STAGE AIR COMPRESSION 89 

The total power required for the above compressor at the 
speed given is: 

35 1b. 10.17+14.34 = 24.51 
70 1b. 10.17+24.98 = 35.15 
120 1b. 10.17+33.77 = 43.94 

The volume of free air compressed and delivered at either 
pressure is 262 cu. ft. per minute. 

The loss by friction in a two-stage compressor should be 
greater than in a single-stage compressor of the same free air 
capacity and working to the same pressure, and the total fric- 
tion of single-acting cylinders must be proportionately greater 
than that of double-acting cylinders, so that if for a common 
single-stage double-acting compressor we allow 10 per cent, for 
the total friction of the machine, it is probable that 15 per cent, 
is not too great to allow for the arrangement that we have been 
considering above. 



CHAPTER VIII 
TWO-STAGE AND THREE-STAGE COMPRESSION 

There may be those who could think that in the preceding 
chapter most of the space had been wasted, inasmuch as the 
matter presented was more in the nature of an explanation of 
''how not to do it/' as any case of single-acting, two-stage 
tandem compression must be. Having gone so far, however, it 
may be proper to go a little further to satisfy ourselves as to 
the advantages or otherwise in the double-acting tandem 
arrangement. 

Having the single-acting, two-stage tandem arrangement 
still in view, it is to be noted that when the compressor is in 
operation both pistons are always exposed to atmospheric pres- 
sure upon the sides nearest to each other. The other side, or 
the compressing side, of the larger piston is also exposed to 
atmospheric pressure, or very near it, during its intake stroke. 
The compressing side of the smaller piston is never exposed to 
atmospheric pressure when the compressor is in operation. 
During the intake stroke of the smaller cylinder, while it is receiv- 
ing the air which is being compressed in the larger cylinder, its 
piston is subject to the pressure that is due to that initial com- 
pression. As both of the pistons are upon one rod, whatever 
pressure there may be against the smaller piston when the larger 
piston is doing its work is just so much help for the larger piston, 
and consequently chde of Fig. 19 is to be deducted from the total 
work of Fig. 17. 

In Fig. 20 the area cbde, representing this reacting pressure, 
has been reduced to the scale corresponding to the relative area 
of the larger cylinder, and has been superimposed upon Fig. 17. 
It will be seen that until the point i is reached the steam cylinder, 
or whatever motor is employed, has ''less than nothing" to do, 
and if the compressor were running slowly, it would be apt to 
give a perceptible jump ahead just after passing this center. 
This has been actually observed to occur in a compressor of this 
type. In Fig. 21 the two diagrams have been combined into a 

90 



TWO-STAGE AND THREE-STAGE COMPRESSION 91 

single figure, with AB as the line of no resistance. This, it will 
be remembered, represents the distribution of the resistance for 
the compression-stroke of the larger piston. For nearly one- 
quarter of the stroke, considering here the air-cylinders only, 
and with no reference to the driving power of the steam-cylinders, 
the larger piston has a force behind it greater than the resistance 




Fig. 20. 




Fig. 21. 




Fig. 22. 




Fig. 24. 
Distribution of the Load. 



in front of it. From the point i the net resistance begins to rise 
before the larger piston, and continues to rise until the ex- 
treme end of the stroke, except for a slight interval at the middle. 
Fig. 22 represents the resistance for the return stroke, which is 
the delivery-stroke of the smaller piston. This diagram is the 
same as baed of Fig. 18, but drawn to the scale of the larger 



92 COMPRESSED AIR PRACTICE 

cylinder for comparison. It has also, for convenience of com- 
parison by the eye, been reversed endwise. 

It is easy enough by a glance at Figs. 21 and 22 to see the dif- 
ference in the resistances for the alternate strokes. If the com- 
pressor were delivering the air at 35 lb., instead of at 70 lb., the 
upper line of Fig. 22 would approximately follow the dotted 
line ba, and the resistance would be practically uniform for the 
entire stroke. Fig. 21, representing the alternate stroke, would 
remain precisely the same whether the smaller cylinder were 
delivering the air at 35 lb., at 70 lb., or at any higher pressure, 
and even at the lower pressure the resistance for this stroke would 
not be as great as for the delivery-stroke. 

It is evident that the resistance for the alternate strokes could 
not be equalized by changing the relative capacities of the two 
cylinders. To reduce the smaller cylinder would indeed tend 
toward an equalization of the resistances by allowing the first 
cylinder to do more work and compress the air to a higher pres- 
sure; but to raise the pressure in the first cylinder would be to 
defeat the purpose for which the two-stage compression is adapted, 
that of allowing a cooling of the air and a reduction of its volume 
before its compression is too far advanced. 

As Figs. 21 and 22 represent the resistances for the alternate 
strokes of single-acting cylinders, these resistances may be added 
together and we may combine them, as is done in Fig. 23, and 
we then have the diagram for either stroke of tandem double- 
acting cylinders of the same sizes. This of course represents 
double the free air capacity of the single-acting cylinders. Fig. 
24 is a theoretical diagram of a double-acting single-stage com- 
pression cylinder of the same capacity, the assumed compression- 
line being the mean of the adiabatic and the isothermal curves. 

The maximum resistance for the stroke in the two-stage double- 
acting compressor is only three-fourths of the maximum resist- 
ance for the single-stage compressor. The resistance at the be- 
ginning of the stroke is not as low in the former as in the latter, 
and the distribution of the resistance over the whole stroke is 
decidedly more uniform. As to the total effective resistance 
for the stroke, as we have here developed it, the two-stage, com- 
pressor shows no advantage over the single-stage even while 
ignoring the additional friction of the former. In fact, the mean 
effective resistance of Fig. 24 is somewhat less than that of Fig. 
23. This might have been expected, because in the cylinders 



TWO-STAGE AND THREE-STAGE COMPRESSION 93 

from which Fig. 23 was evolved the full benefits of water-jacket- 
ing were not employed, the cylinder heads, for instance, not 
being jacketed at all. 

Two -stage Cross Compound. — The indicator-cards, Figs. 25 
and 26, though in several particulars not ideal, still speak for 
themselves as to the equitable distribution of load which is 




Fig. 25. — Low Pressure, Two-stage Double Acting Cross Compound 

Compressor. 

possible in two-stage compression, without inviting any ques- 
tioning such as the machine with the single cylinders provoked. 
In this case each double-acting air cylinder is tandem to a 
steam cylinder of a cross-compound machine, the cranks of 
which are at right angles, the air between the stages passing 




Fig. 26. — High Pressure, Two-stage, Double Acting Cross Compound 

Compressor. 



across from the low-pressure to the high-pressure cylinder through 
an efficient intercooler which with its connections has a large 
air capacity as compared with the cylinder volume. 

The low-pressure air-cylinder was 31 in. in diameter and the 
high-pressure cylinder 19.5 in. in diameter with a stroke of 42 in. 
and at the leisurely speed of 40 revolutions per minute h piston 



94 COMPRESSED AIR PRACTICE 

speed of 280 ft., which is not more than one-half the speed at 
which n?any compressors are run to-day. The scale of the 
first card is 20 and that of the second card is 60. 

These cards taken together represent a remarkably even dis- 
tribution of the load throughout the entire revolution. A little 
over one-half of each stroke of each cylinder is consumed in 
con: pressing the contents while the other half of the stroke is 
occupied in expelling the charge at the full working pressure for 
that cylinder. The compression portion of the stroke of one 
cylinder is coincident with the delivery part of the stroke of the 
other cylinder, and these occur together for each quarter of the 
revolution, so that the fluctuations of torque are much less on 
account of the air-cylinders than for that of the steam-cylinders, 
and the air cyhnders thus help to promote rather than to defeat 
steady running. 

The low-pressure cjdinder delivers air to the intercooler for less 
than one-half of each stroke, while one or the other of the ends of 
the high-pressure cylinder is taking air nearly all the time, but 
the fluctuations of intercooler pressure are almost imperceptible, 
and all the air passing through is at practically the highest inter- 
cylinder pressure, or in the best condition for cooling. 

A Study of Three-stage Compression. — Having the formulas 
provided, with the detailed requirements as to volume of air, 
ultimate pressure, etc., it would seem to be a simple thing to 
compute the cylinder dimensions and other particulars and then 
to proceed to design and build the machine. Before we get 
very far, however, we find that there are many details to be de- 
cided upon, possible different arrangements to be selected 
from, with no absolute best in sight. 

The compressing of air is not in any case as simple an operation 
as the pumping of water, and when high pressures are required, 
involving, as in this case, multiple-stage compression, the problem 
of equitably apportioning the work and the effect for each step 
of the operation, the providing for the easy flow and the efficient 
cooling of the air between the stages, the reduction of machine 
friction to a minimum, the providing for the proper lubricating 
of all the working parts, the arranging of all for ready accessibility 
when wear or accident makes it necessary, constitute in all an 
intricate problem, the solution of which is well worth looking 
into. Nothing is final, and everything achieved is a challenge 
to surpass it, so that doubtless later, or even now, there may be 



TWO-STAGE AND THREE-STAGE COMPRESSION 95 

a better machine than this we have before us, or than any we 
now know of, and we must note its points of excellence while we 
may. 

There is that which is picturesque for the engineer as well as 
for the landscape gardener, and here, Fig. 27, is a picturesque and 
interesting machine, embodying a number of details of ingenious 
design well worth considering. It is a three-stage air-compressor 
designed to work to 1000 lb. gage pressure, with a free air capacity 
of about 50 cu. ft. per minute. It is a power-driven machine, 
the type of power application being according to circumstances. 
As here shown it has a pulley for a belt drive, but it may also be 




Fig. 27. — Section of Three-stage Belt Driven Compressor. 

driven by a chain, by gearing, a Pelton wheel on an extension of 
the shaft, or by a direct-connected electric motor. 

The dimensions of the cylinders are 8-, 5- and 2 3/8-in. di- 
ameter, respectively, by 8-in. stroke, and the normal speed is 150 
r.p.m., giving a piston speed of 200 feet. The low pressure or 
intake cyhnder is double acting and the other two are single 
acting. The three cylinders are in a straight axial line, one 
piston rod extending from the cross head through all the pistons. 
The low-pressure cylinder is between the other two, the inter- 
mediate cylinder being in front, or toward the crank, and the 
high-pressure cylinder at the back. The pistons of the inter- 
mediate and of the high-pressure cylinder are, in fact, plungers 
on each side of the low-pressure piston. The working area of the 



96 



COMPRESSED AIR PRACTICE 



low pressure piston on each side is therefore, although we call it 
double acting, only that portion of the piston which surrounds 
the plungers, and these areas are quite different on the two sides 
of the piston on account of the different plunger diameters. This 
arrangement gets rid of all cylinder heads or partitions and stuff- 
ing-boxes between the cylinders. The only stuffing-box for the 
entire machine is that in the head next to the cross head and 
opposed to the working pressure of the intermediate cylinder. 
The piston rings are the only packings required besides this one 
stuffing-box. The packing rings for the low-pressure and the 
intermediate pistons are sprung into grooves turned in the soHd 
metal. In the high-pressure piston or plunger the grooves are 
not turned in the soHd but are formed by removable sections which 
fit the piston-rod and also the cylinder bore, and which are cut 
away at the outer corner to form the grooves for the rings. 
When the main portion of this piston is in place in the cylinder, 
a packing ring is slipped in to fit against it; one of the movable 
sections of the piston is then slipped in against th^'s, then another 
ring, then another piston section with a washer and nut outside, 
which secures all. The middle or low-pressure piston has a taper 
fit on the rod, and is secured by a nut outside the intermediate 
plunger, which thus locates it precisely and holds it securely. 

The actual working clearances of one of these compressors, 
as determined by the inspector, were: For the low-pressure 
cylinder, 3/32 in. and 3/32 in.; intermediate cyUnder, 1/8 in.; 
high pressure, 1/8 in. 

The distribution of the pressures and the apportionment of the 
work of compression throughout the cycle of operations of this 
compressor are such as to make the work for each stroke nearly 
the same. On the forward stroke, or with the pistons moving 
toward the crank, the low-pressure piston and the intermediate 
piston are both compressing to their respective pressures, and 
the second intermediate pressure against the high-pressure 
piston is assisting; that is, its thrust, at practically constant 
pressure, is to be deducted from the total work done by the other 
two pistons. 

On the backward stroke the low-pressure and the high-pressure 
pistons are compressing, and the first intermediate pressure 
against the intermediate piston is assisting in the work. By 
calculation, based on the assumption that the compression is 
adiabatic; it is found that the horse-power required for the 



TWO-STAGE AND THREE-STAGE COMPRESSION 97 

forward compression stroke of the low-pressure cylinder is 2 . 79, 
and for the same stroke of the intermediate cylinder, 8.52 h.p. 
The total horse-power resistance for the forward stroke is 8.52+ 
2 . 79 = 11 . 31 h.p. The intake pressure against the high-pressure 
plunger which is approximately constant, is suflficient to develop 
3. 13 h.p. which is to be deducted from the working horse-power 
of the other two cylinders, giving us as the net horse-power for 
the forward stroke 11.31-3.13 = 8.18 h.p. For the back 
stroke of the low-pressure piston the horse-power resistance 
equals 4.17 h.p., which is greater than for the forward stroke, 
because of the increased piston area. The resistance for the 
working stroke of the high-pressure cylinder equals 8.93 h.p., 
giving a total for both the low- and high-pressure cylinders of 
4.17 + 8.93 = 13.10 h.p. From this it is necessary to deduct 
the power due to the intake pressure against the intermediate 
piston, which is found to be 3.13 h.p. The net power for the 
back stroke then will be 13.10-3.13 = 9.97 h.p. 

While the work of the two strokes is so nearly equal, that 
of the back stroke is the larger, which is as it should be, as this 
occurs on the thrust of the connecting-rod instead of on the pull. 
A sufficient reduction of the terminal delivery pressure would 
equalize the work for the alternate strokes. 

As before stated, adiabatic compression is assumed in each 
cylinder, with efficient intercooling between the stages. The 
cylinders are all very completely water-jacketed, so that the 
temperatures of the working surfaces are kept down and satis- 
factory lubrication is maintained, but there is little cooling 
effect upon the body of air in the cylinders during the operation 
of compression. 

The circulation of the cooling water is accomplished by a 
single continuous flow through both intercoolers and all the 
water-jackets, there being no places where the water can remain 
without change. The efficiency ot the intercooling is sufficiently 
indicated by the fact that the temperature of the air leaving 
the second intercooler and entering the last compressing cylinder 
was found upon a prolonged test to be the same as that of the 
initial intake, or 70°, and the temperature of the air as finally 
delivered, there being no aftercooler, was 188°. The final 
temperature for perfect adiabatic, single-stage compression 
to 1000 lb. would be above 1250°, which would be prohibitive 
in practice. 

8 



98 COMPRESSED AIR PRACTICE 

In a test which was made of one of these machines, in which 
the number of strokes of the compressor required to fill a re- 
ceiver of known capacity was ascertained, the volumetric 
efficiency of the compressor, or the ratio of the volume actually 
delivered to the total piston displacement of the low-pressure 
cylinder, was determined to be 0.927. 

This compressor considered in detail will be found to be an 
extremely simple one when the complication of function is al- 
lowed for. The main frame combines in itself all the particu- 
lars upon which perfect and maintained alignment depends. 
The first cylinder has a true seat against the frame and the other 
cylinders successively locate themselves by their seating, all 
the joints being scraped and the faces going together without 
packing. Every valve is independently accessible by the 
removal of its cap. Taking off the high-pressure cylinder 
(which is no more work than the removal of a cylinder head) 
makes all the pistons easily removable, and gives complete 
access to all the cylinder interiors. 

For four-stage compression the problem of distributing and 
equalizing the load is a simpler one than for three-stage com- 
pression, and it is not necessary to consider it here. 



CHAPTER IX 
AIR-COMPRESSOR REGULATING DEVICES 

The most numerous class of air-compressors in the world, and 
in a class by themselves, are the so-called air-brake pumps on 
steam locomotives, these being automatically started and stopped 
by slight variations of the air-receiver pressure. In the same way 
the small electric-driven compressors employed in connection 
with the air brakes on elevated, subway and surface cars are 
automatically controlled, and, similarly to these, small isolated 
electric-driven compressors distributed along the line of the 
New York subway and elsewhere for switch and signal service, 
are now entirely responsive to the fluctuations of the air con- 
sumption. As these compressors, from the nature of the service 
are required to be in operation only a small portion of the total 
time, and at intervals which cannot be predetermined, it is proper 
that they should be thus stopped and started. 

The stopping and starting of these as the pressure rises or 
falls comprises their entire control, and there is no variation in 
the rate of compression when running. Air-compressors proper, 
which we are chiefly to consider here, are in larger units than these, 
and are designed for nearly continuous service, and the operat- 
ing and control of them is a different matter. The problem of 
air compression as a whole, as applied to what we may call 
regular or standard compressors, is by no means as simple as 
would at first appear. In most particulars it is in sharp contrast 
to that of the pumping of water, which may be said to offer 
almost ideal conditions as to power adjustment and the dis- 
posal of the output. The head against which pumping is done, 
as, say, by waterworks pumps, is usually constant, the resistance 
is uniform throughout the pumping stroke, the cylinder is always 
entirely full of water at both ends, so that clearance has not to 
be reckoned with, and the work of pumping is usually steadily 
continuous, there being generally ample storage reservoirs for 
the water, so that the pump can run right along all day, and 

99 



100 COMPRESSED AIR PRACTICE 

night as well as day, alwaj^s adjusted to its most economical or 
otherwise most satisfactory gait. 

In the case of the air-compressor, its delivery pressure is, 
indeed, expected to be constant, provided that the air is not used 
at a rate exceeding the capacity of the machine, but all the other 
conditions are as different as could be imagined. While the 
terminal or delivery pressure is constant the resistance to be 
overcome by the piston for each stroke begins at nothing and 
gradually increases until full delivery pressure is reached, which 
then continues uniform for the last quarter or third of the stroke. 
There is always a certain clearance-space unswept by the piston 
at the end of the stroke, this diminishing to that extent the 
dehvery volume, and this undischarged air by its re-expansion 
giving a preliminary shove to the piston at the beginning of its 
return stroke. The air is almost never used at a uniform rate, 
while the capacity of the compressor should be sufficient for the 
maximum demand, which would of course imply, if the compressor 
ran steadily, an excess of air at intervals, while the air storage 
capacity provided is always necessarily small as compared with 
the output of the machine, so that in practice it is quite necessary 
that the compression and delivery of the air shall be controllably 
variable within a considerable range of capacity, instead of 
continuing uniformly at the maximum. 

Safety Valve and Throttle Control. — If such control of output 
is not provided, and if the compressor is run right along at full 
speed and capacity, the only thing to do is to trust to the safety 
valve on the air receiver to dispose of the surplus, a wasteful 
practice which no one could be expected to approve of, and which 
few would allow as a frequent occurrence, preferring rather the 
irksome task of watching the gage and manipulating the throttle, 
and then if the machine could not be run slowly enough of letting 
it stop entirely, with usually the work of barring over the center 
to start up again. 

Automatic Speed Regulation. — On steam-driven machines 
all has been done that could be done to regulate the output by 
automatically varying the speed. This has in the nature of the 
case never alone been satisfactory because the range of possi- 
bilities is so limited. On straight line or single-cylinder machines 
the control of speed alone is especially unsatisfactory because the 
machines can never be run very slowly with the load on, and 
they stop on the centers most ignominiously. Duplex machines 



AIR-COMPRESSOR REGULATING DEVICES 101 

can of course be run much slower without stalling, but speed 
control is in any case coming less and less to be employed, espe- 
cially because so many machines driven by electric motors or by 
water-power direct are necessarily run at constant speed. 

Compressor builders have exercised their ingenuity and have 
provided devices various and innumerable for reducing the air 
output of the compressor without allowing it to stop, or indeed 
without changing the speed at all. These have been successively 
described and advocated in builders' catalogues, which descrip- 
tions it is not our function here to reproduce, but some of the 
devices may be mentioned to show the progress which has been 
made in this desirable feature of control. 

The Run Around. — A device which has been used upon a great 
many single-stage steam-driven compressors provides, when a 
predetermined pressure has been reached, for automatically 
opening one or more of the discharge valves in each end of the 
air-cylinder at the same time, thus allowing the air at delivery 
pressure to play back and forth from one end of the cylinder to 
the other, the pressure against the opposite sides of the piston 
being thus for the time balanced, and the power used consider- 
ably reduced, while the delivery of air stops entirely. When the 
unloading occurs it is provided that at the same time the flow 
of steam at the throttle is choked off, so that the crank shaft 
will keep on turning at about normal speed ready to resume 
work when the reverse movement of the unloader occurs. The 
fact that by the action of this device the work is either all on or 
all off, instead of gradually increasing or reducing the load accord- 
ing to the demand, causes the unloading and the resumption to 
occur more frequently than is desirable, and the friction of the 
already heated air in playing back and forth through restricted 
passages must raise its temperature still more and cause trouble 
in that direction. 

Choking the Intake. — In the choking intake arrangement all 
the intake air for both ends of the cylinder comes to it through 
a single pipe, and a valve is placed here for choking or throttling 
the intake according to the air consumption, the valve being 
automatically adjusted by the pressure in the receiver. Nor- 
mally the intake pipe is fully open, and when the receiver pressure 
rises this pressure operates the valve, closing it partially or 
entirely as may be. The operation of the valve is gradual and 
there is no shock or suddenness in either the choking or the 



102 COMPRESSED AIR PRACTICE 

reopening of the passage. The partial vacuum created by the 
choking of course adds to the power required to drive the piston 
as compared to what it would be if running perfectly free. 

The Skip Valve. — When the choking controller is applied to 
the intake of the first or low-pressure cylinder of a two-stage com- 
pressor and the air is wholly or partially choked off by it, the 
high-pressure cylinder still continuing to work at full capacity, 
the pressure in the intercooler is thereby abnormally reduced 
and the air passing through it, not having been sufficiently 
compressed and heated cannot receive its proper cooling effect, 
and also more than its share of the work of compression is thrown 
upon the high-pressure cylinder with an abnormally high tem- 
perature for the air finally delivered. To correct this skip valves, 
are provided at each end of the high-pressure cylinder to allow 
the air to by-pass automatically from this cylinder back to the 
intercooler when the pressure in the latter has been reduced to 
a point to which the valves are adjusted. This arrangement 
equalizes the work of the two cyhnders and keeps the temperature 
of compression down. 

Inlet Valve Step by Step Regulation. — On a power-driven, 
duplex, tandem, two-stage air-compressor; that is, a machine 
with both a low-pressure and a high-pressure cylinder on each 
side, air regulation has been accomplished by the successive 
automatic release or closure of the inlet valves in successive 
pairs, one high pressure and one low pressure at a time. As 
there are four such pairs of valves, the machine is thrown out of 
operation one-quarter of its capacity at a time, while the power 
required though proportionally reduced remains equitably 
distributed for the shaft rotation. All the inlet valves are of the 
Corliss type w^ith releasing gear which does not release when the 
cylinder is normally compressing, the valves opening to give 
free admission for the entire intake stroke, and closing just before 
the compression stroke begins. The action of the unloader for 
either valve is to admit air to a trip cylinder which pushes out a 
plunger that in turn operates a trip cam and releases the hook. 
The valve then remains open and the air plays freely in and out 
of the cylinder, atmospheric air for the low^-pressure cylinder and 
intercooler air for the high-pressure cylinder. There being eight 
Corliss inlet valves they are unloaded in the following order: 

First stage: left-hand low-pressure front end and left-hand 
high-pressure rear end. 



AIR-COMPRESSOR REGULATING DEVICES 103 

Second stage: right-hand low-pressure front end and right-hand 
high-pressure rear end. 

Third stage: left-hand low-pressure rear end and left-hand 
high-pressure front end. 

Fourth stage: right-hand low-pressure rear end and right- 
hand high-pressure front end. 

This gives the following variations of capacity: 

Full-load, 3/4 load, l/21oad, 1/4 load, no-load. At whatever 
stage of release the machine is working the high-pressure and low- 
pressure cylinder ratio is the same, and whatever air is being 
delivered is compressed at full two-stage economy. The throw- 
ing off of a quarter of the load at a time does not produce any 
perceptible rush of current in an alternating-current motor, while 
to throw off suddenly the entire load of a large compressor is apt 
to produce surges in the line. An additional advantage of this 
method of unloading is that at the same time that it reduces the 
power consumption it also reduces the pressure on the journals; 
when the capacity is reduced one-half, the load on the bearings 
also is reduced to one-half, and when there is no-load on the 
cylinders there is no working load on the bearings. 

Controlled Inlet Valve Closing. — On compressors driven at 
constant speed, and with Corliss inlet valves provided with 
Corliss trip valve gear, it is a natural and obvious suggestion to 
control this trip by the varying air-pressure, making the release 
and closure of the inlet valves to occur sooner or later at varying 
points of the stroke and upon the ends of the cylinders in pairs, 
in this way securing a graduated control embracing the entire 
range of compressor capacity. 

Inlet Valve Opening. — Instead of closing the intake valves, 
and so reducing the volume of air taken into the cylinders, 
which air after admission is compressed and delivered, another 
way is to automatically open the inlet valves and keep them open 
either for the entire compression stroke or for any portion which 
may be necessary to sufficiently restrict the compression. As 
long as any inlet valve remains open the air is free to play in and 
out of the cylinder, and while it is thus free to come and go there 
is of course no compression; also only a small portion of the power 
required for compression will be called for, and as the air is con- 
stantly changing it does not overheat the operating parts. 

Clearance Controllers. — Among the latest devices put into 
practical use for controlling the volume of air compressed and 



104 



COMPRESSED AIR PRACTICE 



delivered, especially by compressors driven at constant speed, by 
direct-connected electric motor or otherwise, is the clearancie 
controller. In the use of this neither the intake nor the discharge 
valves are interfered with in any way. There is simply an 
enlargement of the clearance space in each end of the cylinder, 
or, if two-stage, in both ends of both cylinders, so that the air 
which is not required for delivery into the system is merely 
compressed to the required pressure, and then upon the return 
stroke of the piston is allowed to re-expand against the retreating 
piston and give back the power by which it was compressed. By 
this arrangement the additional advantage is that the tempera- 





plj 









Fig. 28. — Section of Two-stage Compressor with Clearance Controller. 



ture of the air is not augmented by the operation, for although 
the usual rise of temperature occurs on the compression stroke 
there is a corresponding fall of temperature upon the return 
stroke, and the water-jacket effect is also continuously operative. 
Connected by large passages with each end of each cylinder 
are two clearance pockets the combined capacity of which, with 
the passages, is just sufficient to contain the entire cylinderful 
of air when compressed to the working pressure without deliver- 
ing any of it, or, preferably, delivering a minute portion only. 
The valves which open or close the communication between 
these clearance pockets and their respective cylinders are con- 



AIR-COMPRESSOR REGULATING DEVICES 105 

trolled by slight variations of the air receiver pressure either way 
from that which the regulating device is adjusted for, and it is 
so arranged that these valves shall open or close in successive 
pairs instead of simultaneously, so that change of air delivery 
will be made fractionally instead of completely. All the valves 
for all these auxiliary clearance pockets are normally closed, 
and the compressor works and delivers the air up to its full 
capacity until some reduction of the delivery is required. 

On either end of either cylinder, if only one of the clearance 
pockets is opened then only that is added to the normal clear- 
ance and one-half of the cylinderful of air will be delivered to the 
air receiver, while if both passages are opened then none of the 
air for that stroke will be discharged. The operation of this 
system of regulation may be assumed in each case to begin with 
the low-pressure or intake cylinder, but when one or both of the 
clearance pockets are opened for either end of that cylinder 
similar pockets are opened for the corresponding end of the high- 
pressure cylinder. 

The half-tone, Fig. 28, shows an end view partially sectionalized, 
of a two-stage compressor equipped with this style of clearance 
control, and in Fig. 29 we have a series of indicator cards show- 
ing the operation of unloading, beginning with full delivery and 
ending with no delivery. 

The cylinders in this case are respectively 27 1/4 in. and 16 1/4 
in. in diameter by 24-in. stroke, driven by a direct-connected 
motor at a constant speed of 150 r.p.m. The cards for the low- 
pressure cylinder are on the left-hand side of the cut, and those 
of the high-pressure cylinder on the right hand, the scale of the 
former being 24, and of the latter 60, although all the cards have 
been reduced (but equally) in the reproduction. The special data 
for each operation accompany the corresponding cards. 

At the top are typical cards familiar to all showing full delivery, 
with the too familiar choppy discharge lines which no one seems 
able to eliminate. 

In the second pair of cards one end of each cylinder is still 
working at full capacity while the other end of each is half un- 
loaded, or the total air delivery is reduced one-quarter. 

In the next pair both ends of each cylinder are half unloaded, 
discharging only one-half their contents for each stroke, or reduc- 
ing the total delivery one-half. 

In the fourth pair one end of each cylinder is half unloaded and 



106 COMPRESSED AIR PRACTICE 

the other end is entirely unloaded, reducing the normal delivery 
three-quarters, and the machine actually delivering only one- 
quarter of its total capacity. 

In the lower pair of cards both ends of both cylinders are en- 
tirely unloaded and no air is being delivered, or so little that it 
is not in evidence. 

Throughout this entire series of regulating changes nothing 

Low Pressure Side High Pressure Side 

Scale 24 Scale 60 

I.H.P. 352.3. H.P Input 388 
r\ /\ Volume 94%, RF. 37 . /\ / '^-„ 

r >-' \, / '^ I ^ \M.E.R 42.18 y -^^M.E.P42.9^> 

. M.E.PI8.9 ^;>\ M.E.P/8.9 j \ ^,''' "--^ 



'■ Vj^J - — -^^-.-/'.T . -/l ■ 

^Z:i"I^^^JO^^ZZZZZZ^ f^v.!! Load 

Full Load LH.R 242.5, H.PInput32S 
Vol. 74%, RE 965 r.-^ 
r"A/\ ,'>^yV -^■.%y^..M.E.R4Z9 I 

', M.E.P l7.04^>':Jo54y «.'"'"^^^-~-~-"'r::~;r..^' 



First Step First Step 

LH.R 181.2, H.RInput22l 

Volume 46%, PE 96 

!^.y^/X ,A,— » <tM.E.p>^^ yM.E.Py 

"^^10.53 ^-^C 10.5 y ^^''^^~ 2>--~^ 



■ ■g^ - . — -w-" ~~~-^ Q 



Second Step Second Step 

I. H.P 1042, H.P Input 158 
Volume 27%. RE 95 

<:TpV M.E.P6.6^,'-;> 

.j4.ERi.e4 ykE>y ^^^Se---^^::^'" 



Third Step Third Step 

I.H.P 31.3. H.R Input 97 • 
Volume 8%. RE 94 

^^^^H.E.P.3.86 M.E.P 386 ,'' 

^''^-.~^nE.pi.84 ^•^•^/fV'" '---^^--'* 







Fourth Step Fourth Step 

Fig. 29. — Indicator Cards Showing Action of Clearance Controller. 

has been done in any way to either the inlet or the discharge 
valves, and they are ready instantly to resume theii functions 
when the clearance pockets are automatically shut off, partially 
or entirety, which will occur when the receiver pressure is lowered 
below the point of adjustment. The thinness of the lower cards 
tells the eye at once how little is the power required when no air 
is being delivered. 



AIR-COMPRESSOR REGULATING DEVICES 107 

The principle of compressor control by the use of clearance 
chambers, with means of opening or closing connection to the 
compressing cylinder, as described above, has suggested the use 
instead of a single clearance chamber for each end of each com- 
pressing cylinder and in full communication with it. This 
chamber, preferably a cylinder, would be of such capacity when 
empty as to contain the entire content of the compression cylin- 
der after compression to the delivery pressure, and in that case 
no air would be delivered, but would be alternately compressed 
and re-expanded with each stroke of the compressing piston. 
Then if this clearance cylinder were fitted with a piston which 
could be automatically advanced or retracted, by the action of 
the fluctuating pressure in the receiver, the clearance capacity 
could be so varied as to change the delivery all the way from 
full capacity to nothing at all, the full delivery occurring when 
the clearance piston had been advanced to its Hmit and the 
clearance reduced to nothing. 



CHAPTER X 
THE DRIVE OF THE COMPRESSOR 

It is not necessary here to give descriptions of the present 
modes of driving air-compressors or to question and discuss 
what drive is best. This will depend upon what means of driving 
are available in the given case, but in a general way it holds true 
that this work like any other should be done as cheaply as pos- 
sible. Having made all reasonable arrangements for econom- 
ically compressing the air, whatever can be saved in the cost of 
the drive should be sought with equal avidity and would equally 
result in proportionate profit. 

And first as to the steam-driven machine. There was a time 
when two-stage compression could be strenuously advocated 
by a builder — which was all right — with a possible power saving 
of 10 per cent, or at the most 15 per cent., while at the steam end 
of the same machine the power cost might be from 50 per cent, 
to 200 per cent, more than it need be. This condition has at 
last been quite generally recognized, and the straight-line, steam- 
driven machines with plain slide valves, little or no cut-off, low 
steam-pressure, no condensation, etc., have given place to the 
Corliss (or equivalent) high-pressure compound condensing 
machines with all the possible incidental steam economics in 
addition to those at the air end. 

Changes in coriipressed-air practice, or, more precisely, in air- 
compressing practice, have been succeeding each other with 
astonishing rapidity since the present century began, and no- 
where more noticeably than in New York City and its immediate 
vicinity. Here compressed air has found a great opportunity, 
and the great contractors, seemingly begotten by the occasion, 
have proved themselves quick learners and then apt exemplars. 
The North River and the East River tunnels and the earlier 
subway work not only provided the ways and means for what is 
best in compressed-air practice, but the magnitude of the under- 
takings made it worth while, and even imperative, to adopt the 

108 




o 

O 

o 

Xi 

a 






THE DRIVE OF THE COMPRESSOR 109 

most reliable and economical apparatus procurable, for the com- 
pressing regardless of the cost of installation. 

Accordingly the several air- compressing plants employed by 
the great contracting companies for the work here spoken of were 
regarded as models of up-to-dateness in every detail which could 
promise economy and precision of working, and both the con- 
tractors who owned and operated them and the designers and 
builders who were their sponsors, all were proud of them, and the 
technical press wrote appreciative descriptions of them as a labor 
of love. 

These were steam-driven plants, with all the accepted steam 
and power economizing devices, which need not here be enumerated 
in detail, and the air ends also provided for two-stage compression 
with efficient intercooling and automatic regulation, which allowed 
no power to run to waste on account of the variations of the load. 

Although the work for which these compressors were employed 
was necessarily of a temporary character there was no tempori- 
zing or makeshift about them, and day by day, comparing their 
performance with that of ''simple" machines which might have 
done the work, they piled up the savings and paid for themselves 
over and over, 

They made such an excellent showing that, humanly speaking, 
we might have said that these machines, when their first job was 
done, would ' be sure of employment again as soon as there was 
more work in their line to be done. They were still practically 
as good as new, so there could be no plea of old age suggested 
against them. 

The first batch of New York river tunnels, the first subways, 
the Pennsylvania Railroad terminal were all completed by their 
aid. Then came the great water tunnel to be driven the entire 
length of Manhattan and across Brooklyn to Staten Island, and 
new subways without end to be built, but for all this later and 
closely following work not one of these compressors of such 
excellent record came into play. 

Electricity Jumps in. — While these fine compressors were at 
work and making such an admirable record other things also 
were working. Especially was electricity having a phenomenal 
business development. Splendidly equipped light and power 
plants of almost unlimited capacity were being installed in New 
York, as well as in every large city, and these, taking advantage 
of every possible economy in steam generating and in power 



110 COMPRESSED AIR PRACTICE 

development, were enabled to offer current for driving air com- 
pressors and for other power purposes at rates which practically 
compelled acceptance. 

The electric drive under such circumstances is an attractive 
proposition. The first cost of the electric-driven compressor, 
including the motor, is much less than that of the steam-driven 
plant, with boiler, condenser, piping and other appurtenances, 
and fuel handling and storage arrangements. The operating 
force required is reduced to one-fourth or less and the ground 
occupied is also minimized, with no exacting requirements as 
to location, so that generally the compressor may be placed 
much nearer the work, with a great saving in the pipe lines. 
The power required is always ready without preliminary notice 
and the power cost stops entirely when the compressor stops. 
The advantage of the electric drive thus offered has been so 
evident from a strictly business and money making viewpoint 
that it could not be ignored, and now electric-driven plants have 
been installed for the extensive new lines of work, because it was 
simply the cheapest thing to do. 

What may perhaps be considered the culmination of the devel- 
opment of the contractors' steam-driven air-compressing plant 
was that at High Falls, New York, for the Rondout siphon and 
adjacent work of the Catskill aqueduct system. Fig. 30. This 
was said to be the largest installation of high-pressure steam- 
driven compressors in the world. It especially emphasized the 
fact that when it com.es to economical working the improvements 
and the savings have been much greater in the steam- drive than 
in the con)pressing apparatus. However apparently temporary 
may be the character of the work, it is found to pay to install all 
the devi(»es and arrangements of economizing function which 
would belong to the most permanent Hues of employment, and 
accordingly here all the familiar steam economizing devices were 
employed, such as high-pressure steam — 150 lb. — compound 
steam cylinders with condensers, feed water heaters, econo- 
mizers, etc. 

It is customary to think only of the saving of fuel in such a 
case as this, but the labor saving also cuts a figure which quite 
compels notice. Here were ten large duplex Corliss, compound 
steam, two-stage air machines running day and night up to 
full capacity, developing over 4000 h.p. and delivering 
24,000 cu. ft. of free air per minute compressed to 110 lb., using 



THE DRIVE OF THE COMPRESSOR 111 

100 to 110 tons of ''birdseye" coal per day, with hand firing; and 
yet the entire operating force, including engineer-in-charge, was 
only eight men for each 8-hr. shift. 

Nevertheless, and notwithstanding the success of this plant, 
if the selecting and installing of it had come only five years later 
we may believe that it would have been a very different plant, 
because by that time the electric companies would have been 
submitting proposals for supplying current. 

In Fig. 31 we get a glimpse of the compressing plant (installed 
1912) of a large contractor for the building of the Lexington 
Avenue subway, New York. It is located at 96th Street and 
First Avenue, close to the East River, and it may be regarded 
as typical of the plants employed upon this line of work at this 
time, there being at the time of this writing a score of compressors 
of this type in operation in Greater New York. 

This plant comprises five electrically driven cross-compound 
compressors with cylinders 25 1/4 and 15 1/4 in. in diameter and 
21 in. stroke, delivering air at 100 lb. with an individual free- 
air capacity at 187 r.p.m. of 2110 cu. ft. per minute or an aggre- 
gate of 10,550 cu. ft. 

These machines have direct connected self-starting synchron- 
ous motors with belted exciters, 365 b.h.p., 6600 volts, 3 phase, 
25 cycle, the rotor mounted on the crank shaft with a bearing 
in each frame, this arrangement giving a greater efficiency than 
that of a high-speed belted motor. The rotor is heavy enough to 
give a sufficient flywheel effect, in this assisted also by the pulley 
which drives the exciter. A speed of 187 r.p.m. — 654-ft. piston 
speed — is high for a compressor of this size, bu it proved practi- 
cable and safe with this type of machine. 

A special conduit running lengthwise of the building is provided 
for the free air supply, the piston inlet providing a ready means of 
connecting. A large intercooler is set transversely above the 
cylinders of each machine, forming the air connection between 
them. In this case special cooling tubes of "admiralty bronze" 
are provided, so that the more or less saline water of the East 
River may be used for the circulation. The air pressure at the 
intercooler is 27 lb. After leaving the intercooler and before 
entering the high-pressure cylinder the air passes through a sepa- 
rator which takes care of all the water liberated in the air by the 
intercooling. 

The compressor runs at constant speed, the air output being 



112 COMPRESSED AIR PRACTICE 

regulated by an automatic clearance controller spoken of more 
in detail in the preceding chapter. Looking over one of these 
machines in detail it will easily appear that a modern air com- 
pressor of the highest type is by no means so simple a machine 
as some might easily imagine, but all the refinements have full 
business warrant for their being. 

From the compressor house the air is carried by a 10-in. pipe 
westward to Lexington Avenue, about half a mile, and then it 
is led away both north and south long and increasing distances 
through 8-in. pipes and smaller. The 10-in. pipe from the com- 
pressor house is laid in the gutter along 96th Street close to the 
curb and carried overhead where it crosses three avenues. In 
the middle of each block a sliding expansion joint is placed, and the 
long vertical pipes and curves at the crossings also yield more or 
less. As the air starts on its journey its temperature is not less 
than 200°, but before Lexington Avenue is reached not the slight- 
est trace of heat remains, which shows how quickly the air cools 
off, and the futility of ever attempting air reheating except close 
to where the air is to do its work. 

From the nature of the work in this case it is impossible to 
make any intelligible comparison between the power used at 
the compressors and the power actually realized where the work 
is going on, as that is so widely distributed, so intermittent and 
of such varied character. The only thing until the work is 
completed is to have enough air at effective pressure always 
ready. 

New York Water Tunnel. — The driving of the great water 
tunnel, in solid rock all the way, from Hill View Reservoir out- 
side the northern boundary of Greater New York, across the 
Borough of the Bronx, under the Harlem River, then the entire 
length of Manhattan, under the East River, across the Borough 
of Brooklyn, under the Narrows and then to Silver Lake in 
Staten Island, the tunnel varying in diameter from 15 ft. to 11 
ft., is being done by sinking shafts, say half a mile apart, to depths 
ranging from 200 ft. to 750 ft. below the surface and then driving 
the tunnel in each direction until a continuous tube results. 
The drive for this work is one or two isolated electric-driven 
compressors at each shaft, these calling for no special comment. 

The electric drive for the air compressor is now common 
enough in every part of the world where the transmission of 
power from distant waterfalls can be made profitable. Such 




> 

< 

a 
o 

bb 



S 

o 
O 
c 



THE DRIVE OF THE COMPRESSOR 113 

drives are now generally preferred and take precedence of the 
direct water-driven compressor, especially on account of the sav- 
ing in the length of pipe lines usually effected. There have been 
in the not distant past a number of interesting installations in 
which a Pilton water wheel has been mounted directly upon the 
compressor crank shaft, and these have usually given good satis- 
faction, but that was before the electric drive had attained its 
present development, and to-day the Pelton wheel which would 
have been driving a compressor direct is much more likely to be 
driving a generator instead for the doing of the same work after 
the transmission. 



CHAPTER XI 
THE TURBO-COMPRESSOR 

The fan blower held its own for so many years, and with so 
little change in design, so little advance in scope and efficiency, 
that it almost seemed a finality in its special field. Those who 
are in the habit of accepting things as they are might for 
many years have considered the fan blower as the one per- 
fected or finished mechanical device, not that its performance 
was so satisfactory, but that there seemed so little promise of 
improving it. 

Its work for so many years was the blowing of blacksmith 
fires and foundry cupolas, with also a lot of ventilation service 
and some conveying of shavings and other light materials, but 
it generally was only worked up to, say, 1 lb. pressure. If higher 
pressures were required the so-called ''pressure blowers,'^ such 
as the Root or the Connersville, came into play, but even they 
did not like to undertake pressures above 4 or 5 lb. 

It is singular, however, that the pressure possibilities of centrif- 
ugal blowers were in sight, at least of the theorists, long ago. 
It was only a question of speeds, the pressure increasing as the 
square of the velocity, and no one had practically approached 
the limit. Table XII (here somewhat changed in form) appeared 
in Appleton's Applied Mechanics in 1878. The velocities here 
given, it is to be noticed, are air velocities and not fan velocities, 
but there are builders of fans to-day who claim and who graphic- 
ally demonstrate that the air velocities are greater than the 
peripheral speed of their fan blades. The table in the latter por- 
tion of it could be only theoretical as it seems to go beyond the 
permissible speed limit. This limit is supposed to be nearly 
reached in some steam turbines which have a peripheral velocity 
as high as 1300 ft. per second. 

The highest air velocity in the table, by the way, is 1016 miles 
per hour. In the United States meteorological records a wind 
velocity of 100 miles an hour is recorded as having been reached 
just once. The pressures being as the squares of the velocities, 

114 



THE TURBO-COMPRESSOR 



115 



TABLE XII.— AIR PRESSURES AND VELOCITIES FROM CENTRIFUGAL 

ACTION 



Pressure, lb. per 


Air velocities, 


Pressure, lb. 


Air velocities, 


sq. in. 


ft. per sec. 


per sq. in. 


ft. per sec. 


0.625 


83 


2 


471 


0.125 


116 


2.5 


527 


0.25 


166 


3 


577 


0.375 


204 


4 


666 


0.5 


236 


6 


816 


0.625 


263 


8 


943 


0.75 


288 


10 


1,053 


0.875 


312 


12 


1,154 


1.0 


333 


15 


1,300 


1.5 


408 


20 


1,490 



only our imagination can suggest what would be the force one 
hundred times as great as that of a hundred-mile wind. 

In all these centrifugal machines — fans, blowers or compress- 
ors — the responsible element, that which fixes the speed limits 
and the pressure possibiHties of the individual units, is the rotor. 
Looking at the rotors of some of the standard types of fans or 
blowers, Fig. 32, they seem to be well designed to fly to pieces if 
run fast enough, and there is a sharp contrast between them and 




Fig. 32. — Typical Rotors of Fan Blowers. 

the rotors of turbo-compressors, as Fig. 33, but these also have 
their speed limits. 

The rotor or impeller in the turbo-compressor as thus far 
developed is generally not left to assume the entire responsibil- 
ity for the compression. It contains vanes which give the air the 
rotation velocity of the impeller before leaving it, but stationary 
vanes are also usually provided for deflecting the air and convert- 
ing its velocity into pressure. In Fig. M, A, A, A are the rotat- 
ing impeller vanes and B, B, B are the fixed deflecting vanes. 



116 



COMPRESSED AIR PRACTICE 



Figs. 35 and 36 will suffice to indicate the essential features 
of the turbo-compressor, Fig. 35 being sections of the impeller 
and its vanes and Fig. 36 a vertical longitudinal section of a 
turbo-compressor unit with five impellers A, A, A. The air enters 
the first impeller as indicated by the arrow, is thrown out into 




Rotor of Turbo-compressor, 



the thin, flat surrounding passage B and then curves back into 
passage C by which it is led to enter the second impeller to repeat 
the cycle as before, and so on. As the air in passage B reaches 
the circumference of the shell, still with a high but somewhat 
diminished rotative velocity, its centrifugal force would oppose 




Fig. 34. — Impelling and 
Deflecting Vanes. 



Fig. 35. — Sections of Impeller. 



its return toward the center, and accordingly at the turn and in 
passage C there are fixed deflecting blades which intercept the 
air, change its direction and convert its centrifugal force into a 
considerably increased pressure with which it enters the second 
impeller. 



THE TURBO-COMPRESSOR 



117 



The spaces D, D, D and E, E, E are filled with cold water in 
constant circulation, and the air passing in comparatively thin 
sheets between these cool surfaces has its temperature much 
reduced. Indeed, except when entering and passing through the 
impeller the air is constantly in contact with these water-cooled 
surfaces and the compression in each successive impeller being 
not sufficient in that alone to raise the temperature very high the 
air emerges from the series at comparatively low temperature. 
When the compression is to high pressures, two or three such 
series as here shown are employed, and the air in passing from 




Fig. 36. — Vertical Section of Turbo-compressor. 



one to the next is carried through an efficient intercooler, and 
throughout the entire compression the temperatures are kept so 
low that the compression is nearer isothermal than with any 
reciprocating machine. 

As early as 1906 there was published a report of the performance 
of a Rateau turbo-compressor designed to compress 1716 cu. ft. 
of free air per minute- to 7.2 atmospheres. At a speed of 4500 
turns per minute, which gave the required pressure, the output 
was 25 per cent, greater in volume than demanded, and on increas- 
ing the speed the pressure was raised another atmosphere. This 
compressor was in four distinct parts, each consisting of a group of 



118 COMPRESSED AIR PRACTICE 

32 impellers in series upon one shaft, with intercoolers between 
the groups. In the first group the pressure was raised from 1 to 
1.7 atmosphere; in the second to 2.9 atmospheres; third, to 4.9 
atmospheres and in the foiU"th to 7.2 atmospheres. The ratios 
of absolute pressures before and after each compression were 
thus successively 0.5882, 0.5862, 0.5918 and 0.6805, which showed 
a remarkably equitable distribution of load for a type of appara- 
tus still novel. 

When we look at Fig. 36 a question naturally arises: The five 
successive impellers as here shown are of the same size, and, being 
upon the same shaft, must have the same volumetric capacity, 
while the actual volume of air entering each successive impeller 
must necessarily be smaller, on account of the immediately 
preceding compression. This evidently is not as it should be, 
and it is not easy to see how the later impellers of the series find 
their proper share of employment or are able to accomplish what 
they should in the way of additional compression. The case is 
apparently not very different from what it would be in a two- 
stage or a three-stage reciprocating machine if all the compressing 
cylinders were of the same capacity. 

The simplest way suggested, and in fact carried into successful 
practice, for correcting this is to make each successive impeller 
thinner than the preceding, or of less volumetric capacity in 
proportion to the reduced volume of air which it is to receive 
from its predecessor. Only the impeller capacities require suc- 
cessive reduction, that of the connecting passages having no 
bearing upon the results after the machine is in full operation. 

It does not appear that changing the diameters of the succes- 
sive impellers, which has been done, could be a satisfactory 
solution. A reduction of peripheral speed would be a reduction 
of compressing force, and, in correct proportion to the volume 
to be compressed, this is as much required for the last impeller 
of the series as for the first if all are to be equally efficient. 

It is not within the scope of the present publication to go into 
details either of design or of construction. Although the essen- 
tial principle of the turbo-compressor is extremely simple there 
are many essential conditions to be complied with. The shaft 
and entire rotor system must be perfectly balanced or the high 
rotative speeds are impossible. Although except the journals 
there are no surfaces to be accurately fitted to work metal to 
metal they must still be adjusted to run very close to each other, 



THE TURBO-COMPRESSOR 119 

and this proximity may lead to actual running contact and 
disastrous consequences if the expansions and contractions due 
to temperature changes are not provided for. The rush of the 
air and the working pressures developed in the running cause 
thrusts in the direction of the axis, which if they cannot be bal- 
anced one against the other must be otherwise taken care of. 

Although the practical value and the high efficiency of the turbo- 
compressor have been demonstrated beyond all question, and 
although many machines are in successful and established opera- 
tion the device as a whole cannot be regarded as beyond the 
experimental stage, and the detailed descriptions of perfected 
machines, as perfection goes in such lines, still belong to the 
future. 

Air-compressor history has been made with great rapidity in 
the last twenty years or so, and even now it may be that another 
chapter as important as any is being added. It is less than a 
quarter of a century since the present writer first came into per- 
sonal contact with a practical, commercial air-compressor of the 
period. It was for such air-pressures as were then demanded for 
the running of rock drills employed in subterranean work, and 
there was then little demand for compressed air for any other 
purpose. The crude and uneconomical compressors first ac- 
cepted were generally superseded after too many years by steam- 
driven machines which no one had any reason to be ashamed of, 
and now these have been driven out of employment by the elec- 
tric drive, as spoken of in the preceding chapter. 

Now what if, when the reciprocating compressor, whatever 
may be the drive of it, has been brought to its highest state of 
efficiency and reliability, it is to be quite generally superseded? 
This seems to be the almost universal law of inventional progress. 
We get all that we can out of a given device, explore and develop 
all its possibilities, begin to think it nearly perfect in its line, and 
then find it crowded out of service when at its best by some other 
thing which is just beginning its course triumphs, it also to be 
knocked out later by some as yet unknown successor. 

There can be little room for doubt as to the great future await- 
ing the turbo-compressor, especially for the larger units. There 
are actually being built at this writing single turbo-compressors 
with capacities of 50,000 to 70,000 cu. ft. of free air per minute 
delivered at a pressure of 3 atmospheres, and in smaller units, 
but still exceeding the capacities of the largest reciprocating 



120 COMPRESSED AIR PRACTICE 

machines, that are capable of delivering air at pressures above 
10 atmospheres 

The two great power companies on the Rand, South Africa, 
have at present in operation six steam-driven turbo-compressors 
of 4650 h.p. each and six electric driven of 4300 h.p. There 
are also building for these companies three steam-driven turbo- 
compressors of 9500 h.p. each, the aggregate of these being 82,200 
h.p. and the air is delivered at pressures suitable for driving rock 
drills, hoists, pumps and general mining service. 

The turbo-compressor seems to offer the ideal conditions for 
direct driving by either steam turbine, electric motor or Pelton 
wheel, although the latter has as yet made little record. The 
ultimate efficiency compares well with that of the best recipro- 
cating machines. Satisfactory means of control is provided 
either for constant volume with varying pressure or for constant 
pressure and a fluctuating rate of consumption. The special 
adaptation of the turbo-compressor seems to be in the furnishing 
of a constant flow of air for blast-furnaces. 

A Mechanical Wonder. — One of these machines which has 
taken the place and does the work of a large Corliss, cross-com- 
pound, condensing, two-stage air machine, maintaining a constant 
supply of air at 90 lb. for the various uses of a large manufacturing 
establishment, I am inclined to regard as in one particular the 
greatest mechanical wonder of the century; and that is in its 
absolute noiselessness. While it is working along steadily, at 
full speed and pressure, any one standing near it fails to discover 
by the ordinary evidence of the senses, by eye or ear or by the 
touch of the hand, that there is any life in it, and only by going 
around the end of the machine and watching the running of a 
worm wheel which has to do with the governing apparatus can 
any assurance of motion and work be obtained. 

An important particular to be noted in connection with the 
employment of the turbo-compressor for general compressed-air 
service is the condition of the air delivered. It at no time gets 
very hot and is finally discharged at temperatures as low as that 
of the output of any other type of compressor. The air as deliv- 
ered has its usual accompaniment of moisture, which may be got 
rid of by the usual separating and draining devices, so that the 
air when used is as dry as when compressed by any other means. 
The air is delivered clean and without any trace of oil to be 
deposited in receiver and piping, so that with the turbo- com- 



THE TURBO-COMPRESSOR 121 

pressor the too familiar ignitions and explosions will be 
impossible. 

It seems to be the general impression that the turbo-com- 
pressor is to find employment only or chiefly in exceptionally 
large units, that in capacity it is to be and to remain the Jumbo 
among compressors. The present writer is under the impression 
that this may not be so. What if at the present time the builders 
of centrifugal air-compressing apparatus may be all on a still 
hunt for the smaller or general compressor trade, and all on the 
quiet expecting to knock big holes in the reciprocating compressor 
business? Quite recently a personal letter was addressed to three 
firms understood to be most prominent builders of apparatus in 
these lines requesting them to kindly send what printed matter 
they were known to be giving out relating to centrifugal blowers 
or compressors for pressures above or considerably above those 
of the ancient fan blower which are measured by inches of water. 
The letters were courteously answered and in each case there 
was sent precisely what was not asked for, which carried not a 
particle of information of the character desired, and what was 
wanted was extracted from these parties by getting business 
friends to pose as possible customers to whom the printed matter 
wanted was forwarded by return mail. 

Those who are afraid that competitors will find out something 
should read KipUng: 

''They copied all I could follow, 

But they couldn't copy my mind; 
And I left 'em swearing and stealing 
A year and a half behind." 

There may be a very different explanation of the situation. 
The apparent reticence of builders as to turbo-compressor 
developments at the present time may be because none of them 
have achieved such success that they are able to brag about it. 



CHAPTER XII 
THE TAYLOR COMPRESSOR— THE HUMPHREY PUMP 

While we are considering the various means and methods of 
compressing air for industrial purposes we cannot omit some 
notice of the Taylor compressor. This is a perfectly practical 
device which has had but a few actual installations, but always 
with complete success. 

The essential principle of the Taylor compressor was known to 
the ancients. The general proposition as embodied in the Taylor 
development would seem to be somewhat paradoxical. Given a 
decided fall of water in a rapidly flowing stream, no matter what 
may be the actual height of the fall, and it may be made to com- 
press air to any pressure desired. 

And yet there is not the slightest suggestion of ''perpetual 
motion" or of ''something for nothing" about it. The work 
done, represented by the quantity of air compressed to the given 
pressure, will always be less by a certain percentage of inefficiency 
than the potential energy of the water which has flowed to do 
the work. 

It is not easy to determine the precise mechanical efficiency 
of this device although it is claimed to have been done, and figured 
statements of efficiencies are matters of record; but it happens 
that in this case the matter is not important, for it seldom can 
happen that the flow of the stream where it may be installed 
will be constant and that the "compressor" will use exactly 
all of it. 

The Taylor air compressor is a device quite similar to what we 
might assume the air lift to be with its operation reversed, both 
being dependent for their action upon the diffusion of air through 
a moving column of water, in the latter case the expansion of the 
compressed air serving to raise the water while in the former 
device the fall of the water compresses the air. 

The most notable Taylor compressor installation is that on 
the Montreal River at Ragged Chutes, near Cobalt, Ontario, 
Canada. There was here a drop in the river of 54 ft. within a 

122 



THE TAYLOR COMPRESSOR 



123 



quarter of a mile, the stream being capable of furnishing 5500 h.p. 
and the "compressor" was designed with a computed capacity 
of 40,000 cu. ft. of free air per minute compressed to a gage pres- 
sure of 120 lb., thus making the compressor at the moment 
easily the largest single compressing unit in the world. The 
location was within reachable distance of a mining location 
capable of using all the air that could be furnished. It is 
transmitted through 9 miles of 20-in. pipe, from the end of 




Fig. 37. — Head of Taylor Compressor. 



which there are two 12-in. branch pipe Unes. About 7 miles 
from the compressor there is another 12-in. branch, making the 
total length of main piping about 21 miles. 

The compressing operation will appear in the progress of the 
description of this interesting plant condensed from the authori- 
tative account of Mr. C. H. Taylor written for Mines and 
Minerals. The water is controlled at the summit level by 
suitable gates. After passing the gates the water flows through 



124 



COMPRESSED AIR PRACTICE 



two 16-ft. diameter intake heads, one of which is shown in Fig. 37 
at a. In each of these heads there are 66 pipes, h, 14 in., in di- 
ameter set in a steel disk c. Below these open pipes the heads 
gradually diminish in diameter until they become 8 ft. 4 3/4 in., and 
from this point they are 15 ft. long. These pipes telescope into 
the intake shafts which are 8 ft. 6 in. in diameter and 345 ft. deep, 
the orifice of the head being at the surface of the water. This 
arrangement permits the heads to be raised or lowered, to con- 
form to the level of the water in the forebay, or the heads may be 




Fig. 38. — Complete Section of Taylor Compressor. 



raised above the level of the water by air hoists, d, thus cutting 
off the supply entirely. The two air-hoist cylinders d act as 
governors, automatically raising and lowering the heads which 
are suspended from them by the hangers, e, thereby regulating the 
flow of water into the intake pipes h according to the demand. 

The water, with the entrained air which has been drawn in 
through the pipes b, flows through the heads with a descending 
velocity of from 15 to 19 ft. per second, this velocity gradually 
decreasing, owing to the compression of the globules of air, and 






THE TAYLOR COMPRESSOR 125 

finally there is a further reduction in velocity owing to the 
enlargement of the last 40 ft. of the shaft, as seen in Fig. 38. 
By the time the water reaches and strikes the steel-capped 
concrete diverting cones a, its velocity is so diminished by the 
baffle from the compressed air that there is little shock. The 
increase of velocity which gravity would cause in falling through 
the shaft does not occur because the force of gravity is overcome 
by the pressure of the air in the chamber or tunnel below. 

The cones a are for the purpose of spreading the flow of air 
and water, thereby bringing the air nearer the flow through the 
tunnel. The air being lighter than the water, it rises to the 
surface of the water, the pressure here being 120 lb. per square 
inch. This tunnel is 20 ft. wide, 26 ft. high and 1000 ft. long in 
the down-stream direction, this unusual length being for the 
purpose of utihzing the total head of the stream, the length not 
being necessary so far as the separating of the air from the water 
was concerned, before the latter started up the outlet shaft b. 
As the velocity of the water is only about 3 ft. per second, prac- 
tically all of the air leaves the water in the first 300 ft. 

The pressure given to the air is due to the height of the body 
of solid water in the outlet shaft, which in this case is 298 ft. deep 
and 22 ft. in diameter, and the air-pressure is therefore constant. 
If the level of the tunnel was higher, making both the inlet and 
the outlet shafts shorter the air-pressure would be proportionately 
lighter, and vice versa. The water fiows along the tunnel and up 
the outlet to the river, the difference in elevation between the 
mouths of the intake and the discharge tunnels being 47 ft., 
this difference in height causing the flow of the water, notwith- 
standing that the air-loaded water in the down shaft is specifically 
lighter than that in the up shaft. 

Near the outlet end of the tunnel its height is increased to 
42 ft., and at this place two pipes are carried through the 30 
degree riser c to the uptake shaft. One pipe d, 24 in. in diameter, 
carries the compressed air to the surface, where it is connected 
with the 20-in. main air-pipe line. The other pipe e is 12 in. in 
diameter and has its lower end submerged at a safe distance 
above the roof of the outlet portion of the tunnel, to act as 
a blow-off in case the air in the tunnel should acquire such pressure 
and volume as to force the water below the level of the tunnel 
outlet. If the air were allowed to escape up the outlet, instead 
of being carried up by the pipe, it would lighten the colunm 

10 



126 COMPRESSED AIR PRACTICE 

of water in that shaft and the air-pressure would not be 
constant. 

The blow-off pipe ends at the upper level of the water in the 
outlet shaft, its end remaining open to the atmosphere. When 
the volume of the air is greater than the demand the air accumu- 
lates in the upper part of the tunnel, forcing the water down and 
exposing the lower end of the blow-off pipe e to the compressed 
air, thus allowing a portion of the water in this pipe to drop back, 
thereby reducing the weight of the remaining water in the pipe 
to less than the pressure of the air. The equilibrium is now over- 
come and the water in the pipes is driven upward to the surface, 
where a most spectacular sight is witnessed, as the body of water 
is shot out by the air sometimes to a height of 500 ft. The 
blow-off continues until the pressure of the air in the tunnel is 
sufficiently reduced to allow the water to again submerge the 
end of the pipe. Water now rises until an equilibrium is estab- 
lished between the air- and the water-pressure in the tunnel. The 
air-pipe and the blow-off pipe are packed in concrete the entire 
length of the 30-degree riser in order to seal them in and prevent 
any escape of air up the outlet shaft. 

It will be seen that the initial cost of the plant at Cobalt, or 
of any similar plant, is not a small one. A writer, apparently 
well informed but perhaps not any too friendly, when the installa- 
tion was approaching completion wrote as follows: 

" The air is conducted to Cobalt by a 20-in. pipe. The pipe was made 
in 40-ft. lengths with welded flanges, and sliding expansion joints set 
in concrete pits every half mile. Aside from the transmission pipe 
lines I would estimate the cost of the plant at the Chutes as not far from 
$1,000,000. This makes the cost per horse-power (5000) about $200, 
which does not compare favorably with the cost of an ordinary air plant. 
I know of two small plants that were installed for less than $90 per 
horse-power including flume and pipe line, as well as wheel and 
compressor." 

The entire compression of the air by this means is almost 
absolutely isothermal, as the air in the down shaft must at all 
times be at the same temperature as the water with which it is 
intermingled. The air delivered, notwithstanding the water 
contact, must be ''dry" air, or air in which there will not be 
sufficient moisture to manifest itself in the subsequent use of the 
air if while it is at its highest pressure and normal temperature 
there is proper drainage of the main pipe line by which the air 
is transmitted. The air also is free from oil, so that there will 



THE TAYLOR COMPRESSOR 127 

be no possibility of ignitions or explosions in receivers or else- 
where, and the air would be almost the ideal, pure, fresh air for 
miner's use except for one unanticipated condition which has 
developed. It is found that the air delivered has less than its 
normal proportion of oxygen. 

It was at once found that it was practically impossible to burn 
candles in the mines when supplied with air from the Taylor 
compressor. A sufficient explanation was furnished at once when 
analysis determined that the air contained only 17.7 per cent, of 
oxygen instead of the normal 21 per cent. The lack of oxygen 
does not apparently trouble the miners, but besides the difficulty 
experienced in keeping lights, the effect of the gases from exploded 
dynamite is much quicker and more serious than was found to be 
the case with air compressed by the usual machinery. 

The oxygen is abstracted from the air by the water with which 
it is in immediate contact during compression, and it appears 
that this result is precisely what should have been expected. 
This matter is quite convincingly explained by Prof. Olin H. 
Landreth, Dean of Engineering, Union College. 

The loss of oxygen, he writes, in hydraulic air compression is 
due to the well-known fact that water will absorb different 
amounts of different gases, just as it dissolves different weights 
of different soluble materials. 

TABLE XIII.— PERCENTAGE OF GASES ABSORBED TO VOLUME OF 
ABSORBING WATER-PRESSURE, ONE ATMOSPHERE 



Temperature deg. C. 


Oxygen, per cent.^ 


Nitrogen, per cent. 





1.027 


1.856 


5 


0.891 


1.630 


10 


0.787 


1.450 


15 


0.704 


1.307 


20 


0.635 


1.191 


25 


0.575 


1.096 



Absorption of Oxygen and Nitrogen in Water. — For oxygen 
and nitrogen, Table XIII gives the approximate percentage by 
volume absorbed to the absorbing water. This is stated for 
normal pressure, or one atmosphere; for other pressures the 
weights of oxygen absorbed increase in approximately direct 
proportion to the pressure, but as the density also increases as 

^ These are true percentages; ratios are 1/100 of the above percentages. 



128 COMPRESSED AIR PRACTICE 

the pressure, the volumes (as measured under the varying 
pressure) are about constant. 

The figures given in the absorption table are for atmospheric 
oxygen and nitrogen, not single, separate gases. They therefore 
represent conditions just as one would find them in hydraulic com- 
pression. The table shows that water absorbs from 80 to 90 
per cent, more of nitrogen from the atmosphere than of oxygen. 

OXYGEN ABSORBED MORE READILY THAN NITROGEN 

If, however, these tabular percentages be divided by 0.21 for 
the oxygen and by 0.79 for the nitrogen, being the proportions 
of each gas in 1 cu. ft. of air, it is seen that for the same volume 
of gas offered for absorption, oxygen is absorbed more than twice 
as fast as nitrogen, and the composition of the air taken up by the 
water is richer in oxygen than the original air. As the absorbed 
air is niostly carried away by the water flowing from the uptake 
shaft, the air which remains and which is used for industrial 
purposes is poorer in oxygen than the original air. The absorbed 
air is largely given up from the water, but only after the water 
has risen in the uptake till the pressure is reduced, and even 
then is largely held in minute globules which give the water a 
milky appearance. 

The Humphrey Pump and Compressor. — The changes which 
are occurring in the means and methods of power development 
are having their effect upon the work of air compression and wider 
departures from established practice are likely to follow. While 
at the beginning the larger air-compressors were quite generally 
steam-driven, many now are operated by gas- or oil-engines, 
saying nothing here of direct and indirect drives by water-power 
and by electricity however generated. The oil-engine working 
mechanically, with reciprocating pistons and rotating shafts, 
is finding a rival for pumping service in the Humphrey pump 
which dispenses with the mechanical devices and yet shows the 
highest efficiencies, using completely all the expansive force of 
the successive charges of explosive mixture. Then, as an after- 
thought or a following up of the invention, the column of water 
set in motion by the explosion, instead of having the further end 
of it driven into a water delivery pipe, is made to expend its force 
in compressing in a suitable chamber, and then expelling into the 
compressed-air system, a volume of air commensurate with the 
disposable force. 



THE TAYLOR COMPRESSOR 



129 



It is not easy to tell the story of the Humphrey pump in few 
words. The following is mostly abstracted from a paper by its 
inventor, Mr. H. A. Humphrey, before the Manchester Associa- 
tion of Engineers : 

The simplest form of Humphrey pump is shown in Fig. 39. 
Imagine a charge of gas and air to be compressed in the top of 
chamber C and fired by a sparking plug projecting through the 
top casting. All valves are closed when the explosion occurs and 
the increase in pressure drives the water downward in C, setting 
the whole column of water in the discharge pipe D in motion. 
The column attains kinetic energy while work is being done on 
it by the expanding gases, and may move with considerable 
velocity when these have reached atmospheric pressure. The 
motion of the water column cannot be suddenly arrested, hence 





=^ 


^-:^^=- 




ET 










Fig. 39. — Simplest Type of Humphrey Pump. 

the pressure in the combustion chamber C tends to fall below 
that of the atmosphere, the exhaust valve E opens, and also the 
water valves FF in the supply tank ST. Water rushes in 
through VV mostly to follow the moving column in pipe D, but 
partly to rise in C in an effort to reach the same level inside the 
chamber as exists in ST. 

When the kinetic energy of the moving column has expended 
itself by forcing water into the high-level tank ET it comes to 
rest, and there being nothing to prevent a return flow, the column 
starts to move back toward the pump, and gains velocity in the 
return direction until the water reaches the level of the exhaust 
valve E, which it shuts by impact. A certain quantity of burnt 
products is now imprisoned in the cushion space F, and the 
energy of the moving column is expended in compressing this gas 
cushion to a greater pressure than that due to the static head of 
the water in tank ET. Hence a second outward movement of 



130 



COMPRESSED AIR PRACTICE 



the column results, and when the water reaches the level of 
valve E the pressure of the space F is again atmospheric, and 
further movement of the water opens valve A against a light 
spring, and draws in a fresh charge of gas and air. If there were 
no friction, the water would fall to the same level as that from 
:which the last upward motion started, but the amount of com- 
bustible charge drawn in is slightly less than this movement 
would represent. Once more the column of water returns under 
the elevated tank pressure, and compresses the charge of gas and 
air, which is then ignited to start a fresh cycle of operations. 

The action of the pump is not altered if, instead of delivering 
into an elevated tank, it discharges into an air vessel, or into an 
open-top standpipe or tower, and both these arrangements are 
useful if a continuous flow from the outlet is desired. 

In the simple form of pump the degree of compression of the 
combustible charge prior to ignition depends on the height to 
which the water is raised, and exceeds the static equivalent of 
the head. This is obvious if one remembers that the kinetic 
energy acquired by the liquid column on its return flow is utilized 
in compressing the combustible gas, while the compression brings 
the column to rest. 

The same considerations enter into the question of the cushion 
pressure attained, but here we are dealing with the compression 
of a volume of gaseous fluid which occupies the clearance-space 
only, and the stroke of the water column is greater in proportion, 
because for the first part of the stroke exhaust products are 
being expelled, and no compression occurs. The cushion pressure 
rises rapidly as the height to which the water is lifted increases, 
and at the maximum lift of about 40 ft. to which the simplest 
type of pump is limited, it may exceed the explosion pressure 
when using producer-gas, or may approximately equal the ex- 
plosion pressure when working with city gas or gasoline. 

A Humphrey pump was exhibited in operation at the Brus- 
sels International Exhibition, and obtained two ''highest pos- 
sible" awards — namely, a Grand Prix in the class for gas- 
engines, and a Grand Prix in the class for pumps. Its construc- 
tion is similar to that in Fig. 39, and explanation is not needed 
till we come to the valve gear shown in Figs. 40 and 41. It 
will be observed that a bolt B sliding horizontally must lock 
either the admission- valve A or the exhaust- valve E by engaging 
under collars a or e, which are fixed on the stems of their re- 



1 



THE TAYLOR COMPRESSOR 



131 



spective valves. Now the bolt is urged right or left, according 
to whether spring Si or S2 is pulling the hardest, and this again 




Fig. 40. 




Fig. 41. — Vertical Section and Plan Showing Valve Gear of Humphrey 

Pump. 



depends on whether the link I, to which the springs are attached, 
has been shifted to the right or left. Suppose the exhaust- 
valve opened last, then its washer m, engaging against cam arm 



132 



COMPRESSED AIR PRACTICE 



p, moves the system p, I, q, so that it leans to the right, in which 
position it is retained by the tension of spring S3. This puts 
tension on spring Si and loosens spring S2; bolt B, therefore, tries 
to move to the right, but until the exhaust valve shuts it can 
only press upon collar e. However, when valve E comes on its 
seat the bolt instantly locks under e, and the same motion which 
holds valve E shut has released valve A, so that next time a 
suction occurs in the combustion-chamber, A only can open. 
Precisely the same kind of action occurs when A shuts and is 
locked and E is released again. Thus valves A and E are auto- 
matically allowed to act alternately, the difference between 





Fig. 42. Fig. 43. 

Figs. 42-43. — Alternative Arrangements of Two-cycle Pump Without 

Valve Gear. 



them being that while E remains open till shut by the rising water, 
A shuts under the action of its supporting spring, so soon as the 
suction in the chamber permits the spring to lift the valve to 
its seat. 

The scavenging-valve V is shown in the plan of the combus- 
tion-head. Fig. 41, and as it operates at the end of each expan- 
sion stroke, its locking and release periods correspond with 
those of the exhaust-valve, and are made simultaneous by a 
lever pivoted at K, and operated by a pin on the bolt B. If 
the water could rush in fast enough when the pressure falls to 
atmosphere, there would be no scavenging action; but the in- 



THE TAYLOR COMPRESSOR 133 

coming water has to be accelerated, and that just gives rise to a 
sufficient suction to effect the desired scavenging. In the ex- 
haust outlet there is a light non-return valve, to prevent burnt 
products being drawn back into the chamber. 

Figs. 42 and 43 show alternative arrangements of the top of 
the combustion chamber for a two-cycle pump requiring no valve 
gear. The combustion chamber has to be specially shaped, 
so that the incoming charge, which may be preceded by pure air, 
displaces the burnt products and mixes as little as possible 
with them. Thus, in Fig. 42, A is the admission valve at the 
top of the tall, narrow part of the chamber B, in which the full 
charge volume extends down to the level cc. A number of 
exhaust valves E lead to a common exhaust outlet 0, which may 
be fitted with a non-return valve, or each exhaust valve may 
carry a light non-return valve on its spindle, as shown. The 
level at which expansion reaches atmospheric pressures is, say, 
//, but this level having been reached by the water, its further 
movement draws in fresh combustible mixture till it occupies 
the space down to cc, and the liquid level has fallen to gg. The 
column of liquid now returns and drives the exhaust products 
through the valves E — which had opened by their own weight — 
until these valves are shut by the water. The kinetic energy 
acquired by the column is now spent in compressing the fresh 
charge, which is ignited to start a new cycle. Thus, each out- 
stroke is a working stroke, and no locking gear is required on 
the valves. 

The same cycle applies to Fig. 43, but in this case there is a 
series of admission valves placed in a ring so as to allow the mix- 
ture to enter with a low velocity in order to prevent eddies and 
mixing with the exhaust products. A higher compression pres- 
sure is obtained with this pump than with the simple pump, and 
consequently higher efficiencies with the same lift. 

Fig. 44 shows the arrangement of a double-barrel pump, 
which has two combustion chambers, A and B, in which explo- 
sion occurs alternately. Any Humphrey pump, whether sin- 
gle or double barrel, may ])e converted into a high-lift pumji by 
means of an air-vessel fitted with valves, and called an "inten- 
sifier." The idea is to first allow the water-column to gain 
velocity, and then to utilize its kinetic energy to (a) compress 
an elastic fluid, and (b) deliver water under the pressure^ to which 
the elastic fluid has been compressed. 



134 



COMPRESSED AIR PRACTICE 



In Fig. 44, A and B are the barrels of a two-barrel pump, 
and at the end of the play-pipe D there are two air-vessels E 
and F, the latter being large enough to give a continuous jflow at 
outlet 0, and to maintain a practically uniform pressure. The 
smaller air-vessel E is fitted with a downwardly projecting pipe 
K, open to the atmosphere at the top, and carrying a valve L 
at its lower extremity, arranged to close under the action of the 
rising water. The cycle starts with explosion, all valves ex- 
cept L being shut, and the water level as shown. While the water 
level in E is rising to L, air is merely being discharged into the 
atmosphere, and as no work is being done by the column of 
water, it gains speed until valve L is shut by impact. Impris- 
oned in E there is now a definite quantity of air, which suffers 
compression until its pressure reaches that at which the high- 
pressure water-valves W can open, and allow the remaining 
kinetic energy of the column to force water into F. Valves W 




f 


.^ 











\ / 


F 


1 






\ £ 


^1 


r^^ 


^^y)^ 


±-- 


r^ 




3 








W 


w 


— 








r 




' 





Fig. 44. — Double-barrel Pump with Two Combustion Chambers. 

close when the column comes to rest, but there remains enough 
energy in the compressed air in E to give, by expansion, the re- 
turn flow, which causes exhaust in A and compression of the fresh 
charge in B to start a fresh cycle. When the water level falls 
below valve L this valve opens, and air is admitted into E for 
the rest of the return stroke. 

Now it is easy to see that if the pipe K is made vertically ad- 
justable with regard to E, the point of the cycle at which L shuts 
can be varied, and more or less air entrapped in E at will. But 
the amount of energy stored in this air will also vary with its 
quantity, for we assume that the degree of compression remains 
constant, and is indeed fixed by the pressure maintained in F. 
Consequently the ratio of the total energy of the working stroke 
to the energy stored in the compressed air in E can be made 
anything desired, or, in other words, we can obtain any compres- 




THE TAYLOR COMPRESSOR 135 

sion pressure of the new charge in B which we like, and this 
independent of the water lift. The advantage is obvious, for 
compression pressures equal to those in modern gas-engines can 
be employed with a corresponding increase in thermal efficiency. 
Further, by manipulating the position of pipe K a given pump 
can be made to meet any conditions as to height of lift, for if the 
lift increases K can be raised, so that the energy stored in the air 
in E remains the same, there being now less air, but at a higher 
pressure. 

An important development of the arrangement is shown in 
Fig. 41, and notice is directed to the fact that at each cycle air 
is drawn into, and rejected from the vessel E. Let us suppose 
K to be connected to a supply of combustible mixture instead 
of opening into the atmosphere, we shall then have an automatic 
pump for taking in mixture and discharging it under pressure. 
If the discharge is into a reservoir from which combustion 
chambers A and B can be supplied, we have at once a means 
of quickening the cycles and greatly increasing the output of a 
given size apparatus. It is convenient to replace vessel E by 
two vessels, one for air and one for gas, so as to maintain the com- 
bustible constituents separate until they enter the combustion- 
chambers. If the first portion of the out-stroke of the water 
column is allowed to reject the surplus air and gas back to the 
sources of supply, then the action throughout the cycle is pre- 
cisely that described when using the single vessel E, except that 
a larger proportion of the total energy is absorbed in the com- 
pression of air and gas, but the excess is given out again during 
the expansion of the pre-compressed charge in either A or B. 
The chief advantage arises from the more rapid working, as 
there is no longer any need to wait for the water-level in A or B 
to fall under the action of gravity when the charge is being 
taken in. In fact, the apparatus becomes practically independ- 
ent of the water-level on the supply side. 

The Humphrey Pump as an Air-compressor. — If the column 
of water oscillating in the play-pipe of a Humphrey pump is 
used as a water piston, and caused to rise and fall in an air 
vessel fitted with suitable valves for the inlet and outlet of air, 
the combination constitutes an air-compressor of a very efficient 
type and promising many advantages. Take the case of a single- 
barrel pump and a single air vessel, shown in Fig. 45. The 
cycle, wliioh may take two seconds to accomplish, is as follows: 



136 



COMPRESSED AIR PRACTICE 



1st out-stroke. 



Pump chamber 
A 

Expansion to atmos- 
phere. Intake of sca- 
venging air. 



1st in-stroke Exhaust till water 

shuts valve e. 
Cushion till water 
comes to rest. 

2d out-stroke Expansion of cushion 

to atmosphere. In- 
take of combustible 
charge in excess. 

2d in-stroke Rejection of surplus 

charge till water shuts 
valve r. Compres- 
sion of charge till 
water comes to rest. 



Air-compressor chamber 
C 

Expulsion of air till water 
shuts valve s. Com- 
pression and discharge 
of compressed air till 
water shuts valve g. 
Cushion till water comes 
to rest. 

Expansion of cushion to 
atmosphere. Intake of 
fresh air. 

Compression of air, but 
not sufficient for further 
delivery. 

Expansion of compressed 
air. 



The flexibility of the air-compressor can now be studied. 
To begin with the pump side, the level of the inlet valve e, 
and the rejected charge r, are assumed to be variable, although 
Fig. 45 being merely a diagram, does not show how the pipes 
carrying these valves are moved vertically. As the level of 





Fig. 45. — Humphry Pump an an Air Compressor. 

these two valves controls the amount of charge ignited at each 
cycle, and the amount of the cushion space, their regulation is 
all that is required to increase or diminish the energy developed 
per working stroke. On the compressor side the position of 
the valves g and s controls the cycle of operations on this side 
of the apparatus, and renders it possible to compress a large 



THE TAYLOR COMPRESSOR 137 

volume of air to a low pressure, or a smaller volume of air to a 
high pressure, or to make any intermediate changes which may 
be desired. 

Thus all the conditions of output, up to the full limit of the 
compressor, may be governed at will, and for all ranges the 
compression pressure of the new charge may be kept up to 
the required degree, so that the apparatus works at its maxi- 
mum efficiency throughout the whole range. The amount of 
water which oscillates between the chambers should theoretically 
be altered along with the total capacity per working cycle, 
but the reason for this is merely to prevent the last portion of 
each down stroke from being wasted by taking in surplus com- 
bustible mixture in one chamber, or surplus air in the other 
chamber, to an undue extent. If the surplus of the combusti- 
ble mixture is unnecessarily large, the extra amount rejected 
will increase the pressure in the reservoir X, and this increase 
of pressure may be made to automatically govern a water supply, 
and so bring up the total volume of reciprocating water, or 
to allow part of the water already in the apparatus to escape, 
so as just to keep a small amount of excess charge for each cycle, 
no matter what may be the output of the pump. 

The Humphrey pump is far beyond the experimental stage. 
It has been built in several units up to 1000 h.p. and its economy 
and efficiency may be said to be fully established. Four Hum- 
phrey pumps, with a capacity of 33,000 gallons per minute each, 
against a head of 25 feet, have been installed and are in successful 
operation at Chingford, England, pumping water from the river 
Lee as a contribution to the water supply of the city of London. 

Not much can as yet be said as to any extensive employment 
of the Humphrey principle to practical air compression, but it 
seemed at least proper to mention it here as among the practicable 
possibilites. 



CHAPTER XIII 
POWER COST OF COMPRESSED AIR 

What is the power cost of a cubic foot of compressed air at 
any given pressure? We will not now look into all the possible 
economies of the case, but will try to get at the actual cost ac- 
cording to the simplest methods of compression. 

Say, then, that we have a steam actuated air compressor, with 
steam-cylinder and air-cylinder both 20 in. in diameter by 24-in. 
stroke, running at 75 revolutions or double strokes per minute, 
using steam at 80 lb. and compressing air to 80 lb. The case 
will then be like this: 

Power required by air-cylinder: 

202-1-0.7854 X 36.6 (mean effective pressure) X 300 (feet per minute 
piston speed) ^ 33,000 = 104.53 h.p. 

Power required in steam-cylinder : 

104.53 + 10 per cent. = 114.98 h.p. 

Volume of free air compressed per minute : 

202 X 0.7854 X 300 -M44 = 654.5 
654.5 — 10 per cent. = 589 cu. ft. free air 

Volume of air when compressed to 80 lb. : 

589X0.1552 = 91.4 cu. ft. 

Power of steam-cylinder (steam 80 lb., cut-off 0.25, mean effec- 
tive pressure or resistance 40.29 lb.) : 

202X0.7854X40.29X300^33,000 = 115.06 h.p. 

Volume of steam used: 

202+0.7854 X 75 -^ 144 = 163.62 
163.62+10 per cent = 180 cu. ft. 

Here 180 cu. ft. of steam at 80 lb. produce 91.4 cu. ft, of air 
at 80 lb., or 1 cu. ft. of air at this pressure costs nearly 2 cu. ft. 
of steam at the same pressure. 

138 



POWER COST OF COMPRESSED AIR 139 

It should be remembered that the same ratio will not neces- 
sarily hold good for other pressures. For lower air-pressures 
the steam will have a little more advantage, and for higher pres- 
sures it will have a httle less. The mean effective resistance as- 
sumed for the air-cylinder is the theoretical resistance with no 
cooling of the air. In practice the actual resistance is somewhat 
less than this, but the difference of mean effective between the 
air card and the steam card, or the friction loss of the machine, 
is also usually more than 10 per cent., so that few of the common 
compressors in use will at their best give any better results than 
the above. 

Table XIV gives the horse-power required to compress 1 cu. 
ft. of free air per minute to a given pressure, also the horse-power 
required to furnish a cubic foot of air at the given pressure; or, 
in other words, the power cost of the operation of air compres- 
sion is exhibited both from the beginning and from the ending 
of it. From either standpoint the power required is given both 
for isothermal and for adiabatic compression, in the one case 
assuming that the air remains at its initial temperature during 
the compression, and in the other case that the air as heated by 
the compression is not cooled during the operation. The power 
required as given in the table is the theoretical power, and no 
allowance is made for the inevitable losses of power that occur 
in its actual application, and, of course, it makes no difference 
what may be the source of the power, or the economy with which 
it may be developed or appHed. The power employed may be 
steam, with or without cut-off or condensation, water power, 
electricity, animal or manual power, or anything else. 

When the volume of free air required to be compressed per 
minute is known, or the volume of air at the given pressure re- 
quired to be furnished, the theoretical power required may be 
found by multiplying the total number of cubic feet required by 
the power required for 1 cu. ft., as here given. In the last 
column of the table, although the compression is assumed to be 
adiabatic, the air after delivery is supposed to have cooled to 
normal temperature, and to have assumed its practically avail- 
able volume, so that the 1 cu. ft. of compressed air represented 
in column 5 is precisely the same as the 1 cu. ft. of column 4. 

Compression Losses in Detail. — In the use of this table the 
second column, showing the power cost of isothermally compress- 
ing 1 cu. ft. of free air to the given pressure, represents the ideal 



140 



COMPRESSED AIR PRACTICE 



TABLE XIV.— HORSE-POWER REQUIRED TO COMPRESS AND DELIVER 
I CU. FT. OF FREE AIR PER MINUTE TO VARIOUS GAGE PRES- 
SURES, ALSO THE POWER REQUIRED TO COMPRESS AND DELIVER I 
CU. FT. OF AIR AT THE GIVEN PRESSURE 





Compressing 


; 1 CU. ft. of 


Delivering 1 cu. ft. per min- 




free air per minute 


ute of air compressed to 


1 


to given pressure 


the pressure given 


Gage 


2 


3 


4 


5 


pressure 


Compression 


Compression 


Compression 


Compression 




at constant 


without 


at constant 


without 




temperature 


cooling 


temperature 


cooling 


5 


0.01876 


0.01963 


0.02514 


0.0263 


10 


0.03325 


0.03609 


0.05586 


0.06399 


15 


0.04507 


0.05022 


0.09105 


0.10145 


20 


0.05506 


0.06283 


0.12994 


0.14829 


25 


0.06366 


0.07422 


0.17191 


0.20043 


30 


0.0713 


0.08464 


0.21678 


0.25734 


35 


0.0782 


0.09425 


0.26445 


0.31872 


40 


. 084305 


0.10324 


0.31375 


0.38422 


45 


0.08954 


0.11166 


0.36368 


0.45353 


50 


0.09508 


0.11952 


0.41848 


0.52605 


55 


0.09936 


0.12702 


0.47112 


0.60227 


60 


. 10402 


0.13418 


0.52855 


0.68181 


65 


0.10808 


0.14028 


0.58612 


0.76079 


70 


0.11245 


0.14718 


0.64812 


0.8483 


75 


0.11629 


0.15373 


0.70952 


0.93795 


80 


0.11926 


0.15971 


0.76843 


1.02906 


85 


0.1224 


0.16555 


0.83039 


1.1231 


90 


0.12558 


0.17096 


0.89444 


1.2176 


95 


0.12886 


0.17629 


0.96164 


1.3148 


100 


0.13121 


0.18153 


1.0243 


1.4171 



and the unattainable, but still the only rational and natural 
standard of efficiency in air compression. Whatever the actual 
power employed may exceed the values in this column is the 
irrecoverable cost of compression. 

In comparing the performance of a steam actuated air-com- 
pressor with this standard we shall find at least four different 
sources of loss in the operation of compression, all enforcing 
some deduction from the ideal efficiency. Few persons in deal- 
ing with compressed air recognize and make the necessary allow- 
ances and deductions for all of these sources of loss, and in con- 
sequence the efficiencies of the air compressors of the day are 



POWER COST OF COMPRESSED AIR 141 

still generally represented to be higher than they actually are. 
In deploring the low ultimate efficiencies in compressed-air 
systems the inefficiencies are to be found lurking at the compres- 
sor end as much as at the air-motor end of the string. 

The first deduction to be made is for the friction of the machine, 
and when indicator- cards are taken this is accurately represented 
by the difference in the mean effective pressures in the air- and 
in the steam-cylinders, assuming the areas and strokes of the 
cylinders to be the same. This difference is often lower than 
might be expected. In some large Corliss compressors, where 
the air-cy finders are tandem to the steam-cy finders, the piston- 
rod of the steam- cylinder being continued into the air-cylinder 
and carrying its piston, the total loss of power in the friction 
of the engine often ranges as low as 5 per cent., where the total 
friction of the same steam-engine if transmitting all of its 
power through its crank shaft would be as much as 10 per cent. 
In the familiar straight line, direct-acting air-compressors the 
friction may generally be assumed as taking 10 per cent, of 
the initial power, and it is seldom lower than that. 

The second source of loss to be reckoned with is in the increase 
of temperature and the reduction of the weight or actual quantity 
of air admitted to the cylinder for compression. This loss is 
seldom recognized, and still more rarely made the subject of 
actual computation. It is difficult to determine it accurately, 
because it is the one detail in the cycle of operations in the 
compressing of air about which the indicator diagram has 
nothing to say. It is evident, however, that there must be 
some loss from this source in any case. As the air is always 
heated when being compressed, and at best only slightly cooled 
during the operation, whatever heat is given up by the air 
is transmitted to the cyfinder surfaces, so that in continuous 
compression they become quite hot. Water- jacketing only 
partially cools these interior cylinder surfaces, and some parts of 
the cylinder and of the heads, with usually all of the piston, are 
not cooled at all by the water. The air, which when heated 
from any source we find to give up its heat so quickly in trans- 
mission, is also heated with equal celerity when the conditions 
are reversed, and it cannot pass through heated passages into 
a heated chamber, which the cylinder is, without being heated 
and increased in volume, so that a less weight or actual quantity 
of air is sufficient to fill the cylinder. The loss from this source 
11 



142 COMPRESSED AIR PRACTICE 

may in many cases be light, but there can be little doubt that 
sometimes it is entitled to a deduction equal to that allowed 
for the friction of the machine. If air whose normal temperature 
is 60° is actually at 120° at the moment when compression 
begins, then the weight of air present is less than 90 per cent, 
of the same volume at its original temperature. 

The third loss of power, or of efficiency, in air compression 
is due to the heating of the air during the compression, and to 
the greater force required for the compression on account of this 
heating. This is the one source of loss that is most generally 
recognized, and too often treated of as the only one. The loss 
in this case is represented by the percentage of excess of mean 
effective pressure above that required for isothermal compres- 
sion. In compressing to 70 lb. the M.E.P. for isothermal 
compression is 26, and for adiabatic compression it is 33.73, 
and the mean of the two is 29.87. The excess of the adiabatic 
above the isothermal is 29.7 per cent., and the excess of the 
mean above the isothermal is still 14.85, or say 15 per cent. 
No compressor within my knowledge does its compression to 
70 lb. with less than this 15 per cent, of loss except by devices 
that increase the friction of the machine, or add to the power 
required or to the cost of operation in some way. 

The fourth source of power loss in air compression lies in 
the fact that w^hile the indicator-cards show, as they do, that 
the M.E.P. for the compression-stroke is above the mean of 
the isothermal and the adiabatic pressures, or when compress- 
ing to 70 lb. more than 15 per cent, above isothermal compres- 
sion, the volume of free air compressed is never a cylinderful. 
The figures in the formulas and in the tables are based upon 
the assumption that a certain volume of air is compressed, and 
when applied to the cylinder of a compressor, the actual capacity 
of the cylinder, or the net area multiplied by the stroke, is the 
volume represented. It is of course the fact that the volume 
actually compressed is always somewhat less than this. There 
is a loss at each end of the stroke. Compression of the air at 
full atmospheric pressure does not begin precisely at the be- 
ginning of the stroke, and all of the air is not expelled by the 
piston at the end of the stroke. It is customary with compressor- 
people to say that clearance in the air-cylinder at the end of 
the stroke does not mean loss of power, but only loss of capacity, 
because the power which has been expended in the compression 



I 



POWER COST OF COMPRESSED AIR 143 

of the air filling the clearance-space is returned to the piston 
by the re-expansion of the air when the piston makes its return 
stroke. The clearance does, however, in effect in our computa- 
tions, represent an actual loss of power, or an expenditure of power 
without any result, because the allowance which the clearance 
demands is so generally ignored, and every stroke of the piston 
is assumed to compress and deliver free air to the full capacity 
of the cylinder, which it certainly never does. 

In practice these four items of loss of power in compression 
occur in different combinations, such as 10, 10, 17, 10 = 60.5 
per cent, net efficiency or 7, 2, 15, 5 = 73 . 6 per cent, net efficiency. 
It is safe to say that the ultimate efficiency never goes as high 
as 80 per cent., while it often goes below 60 per cent. 

And after all this depreciative discourse not a word has been 
said about leakage, which of course would be another detail 
of inefficiency to be added to those enumerated. In air re- 
ceivers and connections, and in piping for transmission, it is not 
proper to befieve that leakage is unavoidable, or that it is more 
than infinitesimally permissible, but in the responsible working 
parts of air-compressors the case is different. There leakage 
may occur, and it would probably be quite an exceptional 
compressor in which there were not some leakage in discharge 
valves, inlet valves or piston, either of which would help to 
reduce the ultimate volume of air compressed and delivered. 
Indicator-cards are so far from intelligibly reporting these 
leakages as something to be corrected that such leakages may 
actually have the effect of producing a better card. Piston 
leakage would make the compression line lower and nearer to 
the isothermal, while discharge valve leakage has been actually 
claimed in specific instances to give the cylinderful of free air a 
higher pressure, or to make the cylinder seem to have taken in 
a greater mass of air ready for the beginning of the compression- 
stroke. We volunteer here no suggestion as to what percentage 
of deduction should be made for valve and piston leakage in a 
working compressor in good condition, but there are undoubtedly 
instances of compressors which have not been properly looked 
after where 5 to 10 per cent, would be warranted. 

Air Consumption of Rock Drills and Pneumatic Hammers. — 
There is a curious thing to be noted in this connection. It is 
generally desirable to know about the rate of air consumption 
of rock drills, pneumatic hammers, etc., especially so that when 



144 COMPRESSED AIR PRACTICE 

a number of such tools are to be put into use it will be possible 
to estimate the compressor capacity required. The air con- 
sumption of such tools cannot be arrived at in advance by meas- 
urement and computation, and such data as we have come from 
records of actual running. In nearly all such records the air- 
compressor has been accepted as the air-meter. The customary 
way is to assume the free air compressed to be equal to the entire 
space swept by the piston, in many instances not even allowing 
for the space occupied by the piston rod, thus making the reading 
of the compressor-meter a considerable percentage too ''fast," 
and the actual consumption of the air-driven tools greater than 
it actually is. The compressed-air output of the compressors 
being thus made to appear too high, the air consumption of the 
drills, etc., has also been rated too high, and one balances the 
other. 

Throughout this chapter we have had in mind the single-stage 
air-cylinder with direct steam drive. Everything would of 
course equally apply whatever the drive might be, whether by belt 
or gear or by direct electric drive, the latter now becoming very 
frequent as a means of utilizing the force of distant waterfalls 
and also in the case of extensive engineering work located near the 
large cities where current is furnished at low cost by the big 
lighting companies. 

Steam Consumption of Air-compressors. — Table XV here 
given was prepared by Mr. O. S. Shantz, M. E., then of Detroit, 
who died in Buffalo in 1910. The diagram. Fig. 46, plotted by 
the present writer, embodies everything contained in the table, 
and may be found more easily usable. 

The table shows the weight of steam required to compress 
100 cu. ft. of free air to the various gage pressures listed, either 
in single-stage or in two-stage compression. They are based on 
the various steam consumptions per indicated horse-power per 
hour shown at the head of each column. Adiabatic compression 
is assumed all through with a power difference of 10 per cent, 
between the steam-cylinder and the air-cylinder or cylinders. 

In using the tables in a given case a steam consumption per 
horsepower- hour is assumed based upon an understanding of 
the type of steam end of the compressor, the steam pressure, 
cut-off, vacuum if condensing, etc. Under this assumed figure 
on the line opposite the required air pressure will be found the 
pounds of steam consumed per 100 cu. ft. of free air compressed. 



POWER COST OF COMPRESSED AIR 



145 




Fig. 46. — Steam Consumption of Air Compressors. 

The corresponding figures at the opposite ends of the diagram indicate 
pounds of steam consumed in compressing 100 cu. ft. of free air to the given 
pressure. 



146 



COMPRESSED AIR PRACTICE 



S « 



I" 



O t» (N t^ (N 



N CO T^ rt< »0 lO 



(N CO 



t^ OS 



lO lO «) O «0 N. t^ 



C^CO CO -^lOiOiOOCOt^t^ 



<N O 
(N 00 



CO Tj) Tjt lO IC 



(M (N CO •»j< rj< »0 IC CO CO ;0 t^ 



e<l (M CO rt< T)< >0 to »C CO CO 



C^C<ICOrl<M<-^U5lO CO CO CO 






_iC_iO CO co_ 
CO 00 o o 



■-H e^ CO coM<Tf(»oio»ococo 



o ^ 

<N t> 

CO CO 



N CO o> 
lO IC lO 



i-i(NcoeoTj(Tf<Tf<ioio»o 



(N CO CO CO •<* ■* 



a>cot^»-HiooO'-iTj<co 

iMC0C0-<i<Tj<T}(iCliOlO 



■-H (N 

O O CO 00 CO IN I> »0 »0 (N t^ 

l>COOO(NCOOCOCOO(Ni* 

i-IINC^lCOCO-^-^TjH-^lOlO 



IC CO CO t^ TJ4 o ■* 
CO (N t>- tH »0 OS (N 



i-H <N CQ C0 CO00-<^Tt<-^lQiO 
00'^rHCDi-HCO00Tt<i-lt^^^ 
IC 1-1 

>-l (N 



CO o 
(N CO 



CO CO Tj< TjH -.^ Tjt lO 



U5 OS 



l-H Tf< t^ OS 



CO CO CO ^ "^ ^ ^ 



.-l<-l<N(NeOCOC0Tt<-*-<*-.# 



CO t^ O CO CO 00 
N N CO CO CO CO 



>-li-((NIN(NCOCOCOCO-<J( 



o o o 

OOOOOOOOOr-i(N 
(NC0-*"5COt*00OS'-Hi-HT-l 

c^^^^c3c3rt^rtc3o3 



QOcooororoDQOQrocoQQ 



^^^^<!<j<!<tJO<i 





■* 


<N 


o 


,_! 


,_! 


o 


,_, 


in 


ID 


o 


IO 


o 






iO 


t^ 


I> 


t^ 


CO 


CO 




OS 


IO 




•* 


CO 


t* 


00 


OS 


o 


^: 


CM 


CO 


CO 


■* 




(N 


I-» 


IO 


rH 


o 


IO 


CO 


OS 


IO 




rH 


OS 


^ 


OS 


CO 


IO 


IO 


■* 


CO 


o 


00 


•o 


CM 




•* 


»o 


t^ 


00 


OS 


o 


^ 


CM 


CM 


CO 


■* 




O 


r-l 




t^ 


CM 


CD 


IO 




f-> 






00 


CO 


00 




<N 


(N 




o 


t^ 


IO 


c^ 


00 




Tt< 


U5 


t^ 


00 


OS 


o 


^ 


»H 


CM 


CO 


CO 














<-H 


1— 1 


1—1 


I-( 








00 


CO 


IO 


IO 


OS 


rH 


IO 


,_( 


CO 


■^ 


IO 1 


t^ 


"-I 




OS 


o 


OS 


OS 


l> 


Tt< 




00 


•* , 




Tt< 


lO 


CO 


00 


00 


OS 


o 


1-1 


cq 


CM 


CO 




00 


,— ( 


Of) 


CO 


CO 


CO 


o 


lO 


on 


CM 


00 


CO 






t^ 


00 


t- 


CO 


IO 




00 


IO 


1-H 


CO 


-* 


IC 


CO 


00 


00 


OS 


o 


1-1 


1-1 


M 


CO 

1-1 

CO 




r^ 


CO 


o 


»o 


C^l 


o 


o 


lO 


IO 


o 


>o 


OS 


CO 


CO 


CO 


IO 


•>* 


CM 


00 


IO 


CM 


l> 




CO 


lO 


CO 


l> 


00 


OS 


o 


o 


^ 


CM 


CM 




lO 


o 


o 


CSl 


t^ 


CM 


CM 


rt* 


o 


■^ 


o 


tH 






-* 


Tt* 


CO 




OS 


IO 




00 


Tjf 




CO 


to 


CO 


l> 


00 


OS 


OS 


o 


m 


^3 


CM 




■* 


CO 


(N 


o 


CO 


»o 


eg 


rH 


o 


<-) 


IO 


£2 


t^ 


o 


(N 




o 


00 


CD 


CM 


OS 


IO 


o 




CO 


K> 


CO 


t> 


00 


00 


OS 


o 


o 


^ 


cq 




,_l 


o 


CO 


o 


o 


o 


-^ 


■* 


CD 


lO 


CD 


M 


CO 


OS 


o 


o 






CO 


OS 






CD 


CO 


CO 


Tt< 


CO 


t^ 


I> 


00 


OS 


OS 


o 


^ 


1-1 




o 


lO 


lO 


CD 


IO 


CM 


IO 


n 


o 


CO 


o 


T-t 


«o 


t* 


00 


l> 




CO 


o 


CD 


CM 


o 




CO 


CO 


■* 


»o 


CO 


l>- 


00 


OS 


OS 


o 


o 


'm, 




o 


o 


IO 


lO 


o 


IO 


IO 


o 


o 


IO 


IO 


o 


Tt< 


CO 


CO 


IO 


00 


o 


t^ 


CO 


OS 


•* 


OS j 


CO 


CO 


•* 


»o 


CO 


t» 


00 


00 


OS 


OS 


o 


o j 




M 


•^ 


CO 


CO 


IO 


r^ 


on 


00 


h- 


o 


on 1 


OS 


(N 


^ 


rtn 


00 


o 


t> 


■* 


OS 


lO 






(N 


CO 


■* 


lO 


CO 


l> 


l> 


00 


00 


OS 


o 


o 1 




J> 


OS 


CO 


o 


o 


o 


IO 


CO 


CM 


lO 


o i 


00 


<— I 


<N 


(N 




00 


IO 


"-H 


CD 


CM 


l> 




(N 


CO 


Tt* 


lO 


CO 


CD 


t^ 


00 


00 


OS 


OS 


o 1 




CD 




00 


o 


CD 


IO 


CO 


CO 


o 


o 


IO 




O 




o 


Oi 


IO 


CM 


00 


CO 


OS 


■* 


00 


CO 


■* 


lO 


IO 


CO 


t> 


1> 


00 


00 


OS 


OS 




■* 


00 


o 


00 


CM 


r^ 


OS 


IO 


00 


IO 


o 


CD 


OS 


OS 


OS 


CO 


CO 


OS 


IO 


o 


IO 


o 


IO 


<N 


CO 


■* 


IO 


CO 


CO 


l> 


00 


00 


OS 


OS 




CO 


(N 


o 


lO 


Of) 


(-1 


O) 


IO 


IO 


(~> 


rH 


S 






t>. 


Tf* 


o 


l> 


CM 


t- 


c^ 


t^ 


1-1 


CM 


CO 




IO 

IO 


CD 


CO 


t> 


t^ 


00 


00 


OS 




CO 


00 


IO 


IO 


o 


IO 








S 


l> 


CO 


lO 


(N 


00 


■* 


o 


Tj< 


OS 


00 


t- 


(N 


CO 


T»< 


IO 


IO 


CD 


t* 


t- 


OS 


00 


°°_ 




O 


<N 


Tt< 


(N 


o 


CD 


o 




CO 


CO 


lO 


CO 


o 


CO 




t- 




IO 


o 


rf< 


(N 


CO 


lO 


IO 

o 


IO 


CO 


CO 


t^ 


t^ 


00 


00 




OS 


1^ 


CO 


o 


CM 


CM 


IO 


CO 




a 


■* 


CO 


^-1 


00 


CO 


OS 


■* 


00 


CM 


CO 


o 


(N 


CO 


■^ 


Tt< 


IO 


IO 


CO 


CD 


t* 


t* 


00 
























'■ 1 
























. 1 


a § 
























fl^ 
























O t- 
























t%. 
























consum 
e-power 


















o 


o 


o 


d 


d 


d 


o 


d 


d 


g 


d 


o 




CM 


D) 


CO 


Tf 


IO 


CO 


t> 


OS 




I-l 




V 


a; 


a) 


0) 


<v 


o 


01 


!1) 


m 


fl) 


aj 












M) 




U) 


bfl 


bl) 


hr 












OS 




crt 


CS 


c! 




s g 


bC 


tli 


tii 


tti 


6U 


U) 


blJ 


M 


bl) 


Wl 


M 


=3 2 


0) 


<a 


<o 


Hi 


<D 


a) 


a> 


(V 


a; 


<o 


<iT 


(h 


IH 


ii 


^ 


(l 


(l 


Ih 


Wi 


Ih 


bl 




If 
It 


3 


3 


3 


3 


3 


3 


3 


3 


3 




3 






































































Ih 


l-l 
















0. 


a 


a 


a 


a 


a 


a 


a 


a 


a 


a 


i 


L 


i 


i- 


i 


i 


i 


i 


i 


i 


i 


:<<<'^'^ :<:<:<:<:<:< | 



POWER COST OF COMPRESSED AIR 



147 





<N 


lOrHC0»Ot^-*00r-llO 

©■^t^oeocDOOr-tco 




lOiOiOOSCOOCOt^t^ 




1-1 
94 


g^Sg?^^gg2 




T}<ioio»ocoocoir)t^ 






53 2 ^ J2 § g? ig j: 8 




Tt<lOlOlOCOCOCDOt^ 




§ 


a><N<Nt-lrH«C00O'-l 

coocooos.-<eocDoo 




Tt<iOiOiOiO<0<OCO«0 




t HC 

2. 


OOOOOt^COO(N»OiO 

Tt<-*io»oioco(r)coco 




OS 


TjJ-^iOU5iOiO«OCOCO 


1 


00 


Tj^Tft-iOOCDOSOOO 
Mt0 05rHTt;?00J-^eC 

■* Tj* Tfi lO IC IC »0 CO o 


00 


C0(N05»O(NrH»O-*T)< 

(Nioi>oeo»cit^05rH 


T 


Tt*Tt(Tf(»0»C»OiOiOCO 


t* 


^OJCO(Nt-OSO5CS>00 

r-ieocoO'-icc>ot>.o> 


o 


■*Tt<TtlTt<lO»OlO»0»0 




SSS^g^^gg 


Oh 


00'*rf<Tt<iO»O>C'O»O 


fe2^S28§S:5S 


coriiy^^^iCiUi\oyr) 




CO 


jeg^g^g^ig^ 


MTj*rt<-*Tt*Tj<iOu:)iO 


Ci.^ 


lO 


SS2JSfejeS^85 


o s 


COC<3Tt<Tj(Tj<rJ4Tj<lOlO 


lO 


Sf:S^5S?2S^ 


COCCfO-<tl-*Tj<Tt<-*lO 


o s, 


■<* 


Tj<coooo(N-*cot^a> 


C0COC»3rf<rf<Tj<Tj<Tt*Tt* 


£ 1 


■<*< 


»0'-H(NMTf<OC0(NC0 

(N>ot^Oi^eo-*«oi> 


WCOC0C0'<J4tJ<tJ<tJ<-.* 


M 


b-OiOOO>»0»00"00 
^«)101>0S^MtH50 


iz; 


eocoMeocoTt<Tt<Tf<-<*< 


8 


eo 


COI^OIOTJ<0»COIO 
OIN-^OOOOi-lCOTt* 




f«5e<3coe»0M^-*Tt<'<j< 


C<l 


ooeoM-iOiooieoco 
os'Heoiot»oooJ'-i(N 


S 


(NeoeocoeococoTj<TH 


^ 


(N 


gS2?;5;S§SS2 


1 


(NcocoeoeoeowMTH 


> 


i 
•SI 

II 

II 
8 k 

is 
i' 




M 




















H 




































iiiiiiiii 

£ £ £ £ £ £ £ £ £ 

S33333333 

o.ao.ao.o.o.o.0. 

:< :^ '^ :< < < '^ < < 





o 


Sosssssag 




OSOO^-liMCVICOeO 




OS 
CO 


OsOso"d-^(N(N(Neo 




00 
CO 


isssssssg' 




«OSOOrHrt(N(N(N 

iCiOi-l>OO»CCO0000 
CONOOOOOSCOt^rHiO 




g 




OOOSOSOO-H;^(N<N 




n 


I0»0t-0'*IOOOOOCD 
tPO»Oi-i<DO"<*<00(N 




OOOSOSOO^^'-C(N 






SS^SJSKSSS 




OOOOOSOSOP'-l^.-l 




CO 


iS§S§^S;3§ 




t^OOOSOSOOO^^ 




?? 


Tt<OSt^CO»OiO<NC>0 

t^fNt^Mt^rHlCOSfN 




t^OOOOOSOSOOO-i 






k0OlC0STj<00(Ni00S 


1 


N.0000000S05OOO 


.-1 

CO 


^s^gssggs 


l>N.0000050S0SOO 


1 


^ 


SSS^S^fegg 


^ 


t-t^l^OOOOOSOSOSO 


s 


COb-b-OOOOOOOSOSCS 


a 


?5 


COM-^iOt^i-llNfOiO 
»00'*OOC4;OOS(N>0 


d 


«Ot^t^t«.000000OSO5 


1 


^ 


MOOt^t>-CO.-Hi-iOO 

eot^r-iioo>cooos(N 


CO CDt^t^t>-000000OS 

THiooscot^oeocooo 


^ 


s 


H 


CDCOcOt^t^OOOOOOOO 




W5 


lOl^lOrHOOSt^lO^ 

OONCDO"*COOS(N»C 




lOCOCOt^t^t^t^OOOO 
COCOOOTt<OOOCO(NI^ 

cooror^-HMcoos-H 




^ 




locococot^t^t^r^oo 




ft o 

a -a 

•a 

i! 

2 .2 

II 




iciocococot^r^t^t^ 




»o»«iiococoeoN.r^t^ 




























Air-pressure, gage 70 

Air-pressure, gage 80 ... . 

Air-pressure, gage 90 

Air-pressure, gage 100 .. . 
Air-pressure, gage 110. . . 
Air-pressure, gage 120. . . 
Air-pressure, gage 130. . . 
Air-pressure, gage 140. . . 
Air-pressure, gage 150. . . 



148 COMPRESSED AIR PRACTICE 

Mr. H. V. Conrad, who first gave publicity to these tables, 
remarked as follows: 

''The accuracy of these tables in practice depends upon the 
correct assumption of the indicated horse-power steam consump- 
tion. Where this cannot be exactly determined the tables can 
at best be considered as only an approximation. The tables 
will, however, be found very useful for quickly making com- 
parisons as to the amount of fuel consumed by the various types 
of air-compressors, thus showing approximately the expected 
yearly saving by the use of, for instance, a compound as com- 
pared with a simple machine. For example, a straight-line 
compressor with a steam consumption of 30 lb., single-stage com- 
pression to 100-lb. gage, requires 9.9 lb. of steam per 100 cu. ft. 
of free air compressed. A compressor with duplex steam cyl- 
inders, at the same steam consumption, but with compound or 
two-stage air-cylinders, requires 8.42 lb. of steam. A compressor 
with compound steam cylinders, non-condensing, with a 26-lb. 
steam rating and compound air-cylinders, requires 7.3 lb. of 
steam, while a high-class Corliss compressor using steam at high 
pressure, with compound steam- cylinders running, condensing 
with a water rate of 17 lb. (including the condenser) and with 
compound air- cylinders, requires 4.77 lb. of steam, or one-half 
as much as in the first example. 

''The average man, however, thinks in pounds of coal rather 
than in pounds of steam. For the purpose of comparison it 
will usually be better, therefore, to reduce deductions to terms of 
pounds of coal burned per hour or per day by dividing the steam 
consumption by 7, since a fair evaporation for average conditions 
is 7 lb. of water per pound of coal burned. This states the case 
upon a dollars and cents basis when the price of coal is known." 

An inspection of the tables will show that two-stage air com- 
pression as compared w^th single-stage is credited, other things 
being equal, with a saving of 15 per cent. 

Referring to the diagram. Fig. 46, the steam consumption for 
single-stage compression is represented by the oblique lines 
which rise from the lower left-hand side, and the lines for two- 
stage compression start from the lower right hand side. The 
numbering at the bottom of the table is therefore repeated in 
reverse order for convenience of reading from either end. For 
general purposes the diagram may be found more convenient 
than, and practically as reliable as, the tables. 



CHAPTER XIV 
POWER FROM COMPRESSED AIR 

When compressed air is used just as steam is used in the 
cylinder of a reciprocating engine, it is quite natural to be com- 
paring the action and value of the two fluids for power purposes, 
although otherwise this book has little to do with steam. In 
general it may be said that no special engine or meter is required 
when air is to be used. Any steam-engine will do, and it may be 
said that the engine may be expected to operate in a more lively 
and frictionless way with air than with steam on account of the 
better lubrication. When air was substituted for steam for 
driving the main hoisting engines at the mines of the Anaconda 
Copper Company, Butte, Montana, larger cylinders were placed 
on the engines, but this was because of the change in the initial 
working pressure. Where steam at 150 lb. had been used as 
more economical the air was to be used at 90 lb., as that pressure 
was better adapted to the other uses of the air in the mine. 

Assuming the air to be used expansively as in the steam-engines, 
the most advantageous point of cut-off being selected and the 
load being adjusted according to the power available, we find 
that a cubic foot of air at any given pressure is not worth as much 
in power developed as a cubic foot of steam at the same pres- 
sure, and the diagram. Fig. 47, shows how this can be so. 

Here we have one volume of steam and the same of air, both 
at 100 lb. and each successively expanded, while doing its work, 
into several additional volumes and until the pressure of each 
falls below that of the normal atmosphere. It is readily seen 
that the two expansion lines are quite different, and that the 
mean effective pressure of the steam is decidedly higher than 
that of the air. 

Thus one volume of steam at 100 lb., represented by the length 
of the line A-1, reaches atmospheric pressure after expansion 
to about six and three-quarter times the original volume, while 
the same volume of air at the same initial pressure drops to the 
same pressure after expanding to a little over four times its 

149 



150 



COMPRESSED AIR PRACTICE 



original volume. The mean effective pressure for the steam, 
taking the entire extent of the diagram, is 27.38 lb., while the 
M.E.P. for air under the same conditions is 19.51 lb., or only 
71 per cent, of the former. As with this cutoff the terminal 
pressures are below the atmosphere, the entire mean effective 
pressures are not properly ''effective" or available or comparable. 
At 1/4 cut-off the M.E.P. for steam is 51.93, and for air it is 
44.19, or 85 per cent., which looks a little better for the air, but 



\ 


L 








, 






Volumes after Expansion 

3 4 5 






t 










r 






8 


















n 
























































































































































































































































































































































































































































\ 
























































l\ 
























































\ 


\ 






















































\ 


s, 
























































\ 


\ 






















































\ 




\ 






















































\ 




\ 






















































s 




N 




















































\ 


s 




"v 


^ 




















































^ 






^ 




St 


e^ 


















1 


























\ 


^ 












^ 














































^. 


^ 


if 










^ 




— 


.«_ 
















































■ 


__ 




— 


— 












■~~ 

















Fig. 47. — Comparative Power Values of Equal Volumes of Steam and of 

Air. 



in this case the terminal pressure of the steam is 11-lb. gage, and 
some of its power is lost through the exhaust. 

This diagram is equally applicable for any other initial pres- 
sure below 100, by taking as the measure of volume the length 
of a horizontal line drawn from the line ^j5 to the expansion- 
line at the given pressure, and taking each repetition of this 
length horizontally as representing an additional volume. Thus 
at 60 lb. pressure 1 volume of steam is represented by 1 1/2, and 
2 volumes would be represented by 3, and at the intersection of the 
vertical line marked 3 we find that the steam pressure has fallen 



POWER FROM COMPRESSED AIR 



151 



to 21 lbs., which is nearly correct. One volume of air at 60 lb. is 
represented by about 1 3/8 of the diagram-spacing, and 2 volumes 
would consequently be 2 3/4 of the spaces, and here we find the air 
pressure to be 13 + , which is the correct terminal pressure for 
air at 60-lb. cut-off at 1/2 stroke, or expanded to double the vol- 
ume. We may take any section of this diagram as representing, 
theoretically, an indicator-card either for steam or air, but we 
cannot take both the steam- and the air-cards and compare 
them by placing one upon the other, because the lengths of the 
two cards will not coincide. 

Fig. 48 is a theoretical diagram, scale 40, showing both steam 




Fig. 48. — Different Expansion Lines of Steam and of Air. 

and air expanded to atmospheric pressure at the end of the stroke. 
In this case the air-line is outside of and above the steam-line, 
and, of course, represents a higher mean effective pressure, but 
it is at the expense of a much larger initial volume. The M.E.P. 
for air filling a cylinder at an initial pressure of 100 lb. for a 
sufficient portion of the stroke and then expanding (without 
loss or gain of heat) so that it reaches atmospheric pressure at 
the end of the stroke will be 41.6 lb. The M.E.P. for steam 
under the same conditions will be 32.46. The volume of air 
used will be .2353, while the volume of steam will be 0.1471. 
If the air gave the same M.E.P. in proportion to its volume, it 
would be .1471: 2353:: 32.46: 51.9, instead of 41.6, and the 



152 COMPRESSED AIR PRACTICE 

greater comparative efficiency of steam under the conditions is 
41.6 :51.9 : : 1 : 1.247, or nearly 25 per cent. 

As the expansion of the air here exhibited is adiabatic, its 
temperature, at least for the latter portion of the expansion, 
would be below that of the cylinder containing it, and the air 
would be heated and expanded, rather than cooled, by its sur- 
roundings; so that there need be no apprehension that the 
expansion-line would be below the theoretical, or that there might 
be still some lurking losses to arise and confront us. The es- 
sential difference in an engine or motor to be driven by compressed 
air instead of steam is a later cut-off for the same initial pressure. 
This later cut-off develops the paradox that although air has less 
available power than steam, volume for volume, the same cylinder 
with the same pressure will develop more power with air than 
with steam, both being used at the point of highest efficiency 
or exhausting at a pressure but little above that of the atmosphere. 

Table XVI shows the mean effective and terminal pressures 
for both steam and air at various points of cut-off and for different 
gage pressures from 50 to 100. Gage pressures are given through- 
out except when below atmosphere when the absolute pressures 
are given in heavy face. It is thought that in this way the table 
will be more serviceable to the general mechanic than if the 
absolute pressures were given throughout. Nothing is said of 
the initial temperature of the air, as that would not affect the 
rate of expansion or the mean effective pressure, after the air 
entered the cylinder. It should not be forgotten, however, that 
re-heating the air just before entering the cylinder would increase 
its volume, and then only a portion of the unit volume assumed 
would be required to do the same work, and in this way the dis- 
advantage as compared with steam, spoken of above, might be 
reversed. 

A single example may be given to suggest one of the many 
practical ways of using this table. Say that we take 1 cu. ft. 
of air at a pressure of 100 lb., cutting off at 1/4 stroke, to get an 
idea what actual power may be realized from it. This might be 
in a cylinder with a piston area of 1 sq. ft., then the first foot of 
piston travel would take the cubic foot of^r to fill the space, 
and a total travel of 4 ft. would allow the air to expand down 
very close to atmosphere^ the M.E.P. for the entire stroke being 
44.19, the foot-pounds of work would be: 144 sq. in. piston area 
X44.19 M.E.P. X4 ft. piston travel = 25453.44. If the air was 



POWER FROM COMPRESSED AIR 



153 



used at full pressure, 100 lb., and without cut-off, it would take 
4 cu. ft. of air to drive the piston the entire length of the cylinder 
and the foot-pounds of work would be 144X100X4 = 57,600, or 
14,400 ft.-lb. per 1 cu. ft., then 14,400: 25,453:: 1: 1.76, which 
means that the air used expansively in this case would do 76 per 
cent, more work than the same volume of air at the same pressure 
without expansion. 

To ascertain the actual air consumption in any case computed 
by the aid of the table at least 25 per cent, should be added to 
the result obtained, this being not too much to allow for clearance, 
leakage and friction. 



TABLE XVI MEAN EFFECTIVE AND TERMINAL PRESSURES OF STEAM AND 
AIR AT VARIOUS POINTS OF CUT-OFF AND FOR DIFFERENT GAGE- 
PRESSURES FROM 50 TO 100 LB. 

All pressures given in the table are gage pressures, except where they fall below atmos- 
phere, when the absolute pressures are given and printed in full face. 





Initial pressure 50 lb. 




Point of 
cut-off 


Mean steam 
pressure 


Mean air 
pressure 


Terminal 

steam 
pressure 


Terminal air 
pressure 


.05 


12.12 


8.87 


2.69 


.95 


^ 


14.39 


10.8 


3.41 


1.31 


.10 


5.44 


1.2 


6.63 


2.54 


1 


8.95 


4.51 


7.13 


3.47 


.15 


10.18 


7.62 


8.65 


4.49 


t\ 


16.55 


11.96 


10.97 


6.14 


.20 


17.9 


13.84 


11.75 


6.74 


.25 


22.83 


18.45 


14.9 


9.23 


.30 


27.11 


23.05 


3.08 


11.93 


i 


29.66 


25.84 


5.22 


13.83 


.35 


30.86 


27.17 


6.3 


14.82 


1 


32.56 


29.07 


7.92 


1.34 


.40 


34.15 


30.87 


9.55 


2.88 


.45 


37.03 


34.18 


12.84 


4.11 


.50 


39.54 


37.12 


16.12 


7.49 


.60 


43.61 


41.98 


22.77 


16.66 


1 


44.44 


42.99 


24.44 


18.53 


f 


45.67 


44.52 


27.24 


21.73 


.70 


46.54 


145.6 


29.49 


24.33 


.75 


47.64 


46.98 


32.88 


28.34 


.80 


48.52 


- 48.08 


36.27 


32.47 


I 


49.43 


49.26 


41.4 


38.85 


.90 


49.64 


49.53 


43.11 


41.03 



154 



COMPRESSED AIR PRACTICE 





Initial pressure 60 lb. 




Point of 

cut-off 


Mean steam 
pressure 


Mean air 
pressure 


Terminal 

steam 
pressure 


Terminal air 

pressure 


.05 


13.99 


10.23 


3.1 


1.1 


T^6 • 


1.61 


12.46 


3.93 


1.51 


.10 


8.58 


3.69 


6.49 


2.93 


1 


12.64 


7.51 


8.22 


4.01 


.15 


16.37 


11.1 


9.99 


5.21 


A 


21.41 


16.11 


1^.66 


7.08 


.20 


22.96 


17.7 


13.56 


7.77 


.25 


28.75 


23.6 


2.19 


10.65 


.30 


33.59 


28.9 


5.87 


13.77 


i 


36.54 


32.13 


8.34 


.96 


.35 


37.92 


33.66 


9.58 


2.33 


f 


39.87 


35.85 


11.8 


3.85 


.40 


41.71 


37.93 


13.22 


5.64 


.45 


45.03 


41.75 


17.1 


10.71 


.50 


47.94 


45.14 


20.91 


13.26 


.60 


52.62 


50.75 


28.59 


21.53 


1 


5.3 . 58 


51.92 


30.51 


23.69 


f 


55.01 


53.67 


33.74 


27.94 


.70 


56.01 


. 54.93 


36.34 


30.39 


.75 


57.28 


56.52 


40.24 


35.01 


.80 


58.29 


57.79 


44.06 


39.78 


1 


59.34 


59.15 


50.07 


47.14 


.90 


59.58 


59.46 


52.05 i 

1 


49.65 



This chapter and that which precedes it have had to do with 
the cost involved in the use of compressed air for power purposes. 
The status of compressed air is not based exclusively or even chiefly 
upon considerations of power cost. It is to be remembered 
that, while compressed air carries with it the tradition of high 
cost, it nevertheless was rapidly extending its field of employ- 
ment when the cost of the air compressed and delivered was two 
or three times as great as at present, and it is no wonder that it 
is still finding increased appreciation and still is extending its 
field of employment. 

Especially does air challenge comparison with steam for the 
operating of the scattered and intermittently operated units of 
a contractor's plant covering a considerable area of operation. 
It is concededly impossible to generate steam at a central sta- 
tion and to transmit it by piping to the several machines to be 
operated, as we do transmit the air, on account of the constant 



POWER FROM COMPRESSED AIR 



155 



Initial pressure 70 lb. 


Point of 
cut-off 


Mean steam 
pressure 


Mean air 
pressure 


Terminal 

steam 
pressure 


Terminal air 
pressure 


.05 


1.06 


11.6 


3.52 


1.23 


^^ 


3.82 


14.12 


4.46 


1.71 


.10 


11.73 


6.19 


7.36 


3.32 


i 


16.33 


10.51 


9.32 


4.64 


.15 


20.55 14.58 


11.32 


5.88 


i\ 


26.26 20.25 


14.37 


8.03 


.20 


28.02 22.06 


0.37 


8.81 


.25 


34.47 28.74 


4.49 


12.07 


.30 


40.07 34.75 


6.65 


0.6 


i 


43.41 ^ 38.41 


11.45 


3.09 


.35 


44.97 40.15 


12.86 


4.38 


1 


47.19 ! 42.63 


14.98 


6.36 


.40 


49.27 


44.99 


17.1 


8.39 


.45 


53.04 


49.31 


21.38 


12.61 


.50 


56.33 


53.16 


25.69 


17.0 


.60 


61.64 


59.51 


34.4 


26.4 


f 


62.73 


60.84 


36.58 


28.85 


f 


64.34 1 62.83 


40.24 


33.03 


.70 


65.48 ' 64.25 


43.19 


36.44 


.75 


66.92 ' 66.05 


47.61 


41.68 


.80 


68.07 ! 67.5 


52.05 


47.08 


i 


69.26 ! 69.03 


58.75 


55.43 


.90 


69.53 69.38 


60.99 j 


58.27 



heat radiation of the line and the consequent losses by condensa- 
tion, besides the trouble caused by expansion and contraction 
of piping, water hammer, etc., the necessity of providing arrange- 
ments for trapping or disposing of the water of condensation, 
and, in spite of all precautions, the frequent stoppages for repairs 
entailed. Each steam operated machine, therefore, must have 
its own boiler and all appurtenances, its own supply of fuel and 
water. Such isolated and intermittently operated machines, 
taking the day through, cost in coal actually consumed at least 
30 pounds and often much more per horse-power hour of work 
actually done, or about twice as much as the coal cost of the air 
operated machines. 

With the air driven machine, when the air is piped to it, that 
ends it, and the operator has only to manipulate the throttle 
and attend to the lubrication. With the steam driven machine 
there is not only the cost of the coal actually consumed, but there 



156 



COMPRESSED AIR PRACTICE 





Initial Pressure 80 lb. 




Point of 
cut-off 


Mean steam 
pressure 


Mean air 
pressure 


Terminal 

steam 
pressure 


Terminal air 
pressure 


.05 


2.72 


12.96 


3.93 


1.39 


t\ 


6.04 


0.78 


4.98 


1.92 


.10 


14.87 


8.68 


8.22 


3.71 


1 


20.01 


13.51 


10.42 


5.08 


.15 


24.73 


18.06 


12.65 


6.57 


i\ 


31.12 


24.4 


1.04 


8.97 


.20 


33.08 


26.6 


2.18 


9.85 


.25 


40.29 


33.89 


6.78 


13.49 


.30 


46.55 


40.61 


11.43 


2.44 


1 


50.28 


44.69 


14.56 


5.22 


.35 


52.03 


46.64 


16.14 


6.66 


1 


54.51 


49.41 


18.5 


7.88 


.40 


56.83 


52.05 


20.88 


11.14 


.45 


61.04 


56.9 


25.66 


15.86 


.50 


64.72 


61.18 


30.48 


20.81 


.60 


70.76 


68.28 


40.21 


31.27 


f 


71.87 


69.76 


42.65 


34.01 


f 


73.68 


71.99 


46.74 


38.68 


.70 


74.95 


73.57 


50.03 


42.49 


.75 


76.56 


75.59 


54.97 


48.35 


.80 


77.84 


77.2 


59.94 


54.38 


1 


79.17 


78.92 


67.43 


63.81 


.90 


79.47 


79.31 


69.93 


66.89 



is also the bringing of the coal to the machine, the supplying of 
the water, the firing and caring for the boiler, with all which 
that implies, so that there is for each machine the labor of a man 
or at least the equivalent of one man's labor to be added to the 
cost of operating. 

The equivalent in coal cost of a man's labor is worth consider- 
ing. Say that coal costs at the machine S4 per short ton. Then 
if the man's wage is $2 per day that will be 1000 pounds of coal, 
or 100 pounds per hour, and for 10 horse-power, which is a big 
allowance for a hoisting engine, this would be 10 additional 
pounds of coal cost per horse-power hour. 

So far, then, as the actual cost of the power used is concerned 
there is evidently a great saving in the employment of air instead 
of steam, and on this account alone it is no wonder that the 
knowing ones choose the air transmission even when there are 



POWER FROM COMPRESSED AIR 



157 



Initial Pressure 90 lb. 


Point of 
cut-off 


Mean steam 
pressure 


,, . Terminal 
Mean air 

steam 

P^^^^^^" { pressure 


Terminal air 
pressure 


.05 


4.59 


14.33 4.34 


1.64 


tV 


8.25 


2.95 5.51 


2.12 


.10 


18.02 


11.17 9.09 


4.1 


i 


23.7 


16.52 


11.51 


5.61 


. 15 


28.92 


21.55 


13.98 


7.26 


A 


35.97 


28.55 ' 2.73 


9.92 


.20 


38.15 


30.78 3.99 


10.88 


.25 


46.11 


39.04 9.07 


14.91 


.30 


53.02 


46.46 14.22 


4.27 


I 


57.17 


50.98 17.67 


7.35 


.35 


59.08 


53.13 19.42 


8.95 


1 


61.82 


56.2 22.03 


11.39 


.40 


64.4 


59.11 24.65 


13.88 


.45 


69.05 


64.45 


29.95 


19.11 


.50 


73.11 


69.19 33.27 


24.56 


.60 


79.67 


77.05 


46.02 


36.14 


1 


81.02 


78.69 


48.72 


39.16 


f 


83.01 


81.14 


53.23 


44.33 . 


.70 


84.42 


82.9 


56.88 


48.54 


.75 


86.19 


85.12 


62.34 


55.02 


.80 


87.61 


86.91 


67.83 


61.69 


7 
8 


89.08 


88.81 


76.1 


72.0 


.90 


89.42 


89.24 


78.88 


75.52 



no special conditions as in mining, tunneling, subaqueous work, 
etc., compelling them to do so. 

In addition to the saving in coal cost there are other advantages 
which air carries with it. In the use of steam there is the time 
taken to fire up and get the pressure before work commences, 
there is the warming up process and the working of the water 
out of the pipes and cylinders every time the machine is started 
up after standing, none of which delays occur with the air, so 
that, in constant readiness and instant realization of power to 
the utmost limit required, the air will every day put in from 10 
to 25 per cent, more actual work per day. Stuffing boxes will 
give no trouble, water will not knock out cylinder heads, pipe 
joints will not be giving out, there will be no chance of low water 
in the boiler, no burning of flues or crownsheet, no possible blow 
up. The cost of repairs, and maintenance will be much less and 
the certainty of continuous readiness for work will be much 

12 



158 



COMPRESSED AIR PRACTICE 



Initial pressure 100 lb. 




greater. While the air driven machines are identical with the 
steam driven type, the individual boilers and all their appur- 
tenances are dispensed with, the cost of them, as far as it goes, 
helping to offset the larger costs of the compressed air installa- 
tion as a whole. The saving in repairs and maintenance with 
this reduction in the cost of the air operated machines may go 
to offset the fixed charges entailed in the larger cost of the com- 
pressors and piping. 






CHAPTER XV 
THE AIR-RECEIVER 

An air receiver is understood to be quite a necessary adjunct 
of any air compressing installation, almost regardless of what 
the air may be used for. It is quite worth while, therefore, 
to consider what may be assumed to be the functions of the air- 
receiver, and how completely or otherwise it satisfies the expec- 
tations and requirements concerning it. We now have in mind 
only compressors and their appurtenances which are installed 
for service sufficiently permanent to warrant the providing of 
safe and economical working conditions. But this should in- 
clude practically all air-compressing plants of any considerable 
capacity, the length of time for which they may be assumed 
to be installed being nearly always sufficient to justify whatever 
will make for efficiency. 

It is gratifying to note how the great contractors of the day, 
as for instance some of those having to do with the Catskill aque- 
duct for the New York water-supply, have learned the economy of 
all expenditures which guarantee ultimate efficiency, and, having 
the courage of their convictions, have equipped and operated some 
of the most perfect air-compressing plants known up to their 
time. 

Compressed air is used as a necessity for operating rock drills 
when sinking shafts, driving tunnels and in general mining 
operations, also in sinking caissons for all kinds of subaqueous 
foundations, etc; but it is also now used as a time and money 
saver, in still greater volume and for a much wider range of 
service when the physical conditions do not compel its use. 
Every extensive manufacturing concern most have its com- 
pressed-air supply distributed throughout the works, and for 
the larger engineering works which are all out of doors, such as 
dam and waterworks constructions and in the most extensive 
quarries, compressed air from a central plant is superseding the 
isolated steam operated pumps, hoists, shovels, stone crushers, 
concrete mixers, etc. 

159 



160 COMPRESSED AIR PRACTICE 

In all these extensive lines of air service, and in fact in all 
air-practice, it makes a great difference as to the condition of 
the air when used, as to the maintenance of constant pressure, 
suitable temperature, and especially as to the moisture which it 
may carry, much inconvenience and loss of time resulting when 
the air is not kept dry and clean. 

There is always a pretence made of attending to this necessity, 
and the air-receiver too often might not untruthfully be said 
to be the embodiment of this pretence. Who would think for a 
moment of installing an air-compressor without a good-sized air- 
receiver as close to it as possible? The man who would set 
up an air-compressor without an air-receiver would lose all his 
respectability among engineers. RespectabiUty, or what cor- 
responds to it, has perhaps a good deal to do with the installa- 
tion of an air-receiver such as it too frequently is to-day than 
anything else, but whatever the ostensible reason for its existence 
it never fails to be more or less of a disappointment. 

If you were to ask the whys and wherefores of an air-receiver, 
you would probably be told that '^ everybody always uses them." 
If you pressed the matter still further, you would most likely 
be told that the receiver is needed for the storage of air and the 
steadying of the flow. Well, the total capacity of the air- 
receiver usually installed with an air-compressor does not exceed 
the output of the compressor for one minute. If the com- 
pressor is running continuously, and if the air is being used as 
fast as it is delivered, the apparatus driven by the air must lose 
its vim instantly and must come to a dead stop within a single 
minute if the compressor stops. So it can be readily seen that 
the air-receiver as a storage adjunct is of little value. 

Receivers do not Cool the Air. — Builders and sellers can afford 
to have you pooh-pooh the storage service of air-receivers, 
but they will still assert the two-fold need of them for coohng 
the air and for getting rid of the moisture in it, the presence of 
the latter being the most serious objection to the air in use 
and the cause of the most trouble. Now as to the cooling 
of the air, how can the receiver do it? To cool the air as it 
comes from the compressor, there must be a cooling surface or 
material for the air to come in contact with, and to which it 
may impart its heat. The air-receiver, however, is a plain 
cylindrical shell with no pretence of a cooling device or any 
arrangement of the sort within it, and the air merely passes 



THE AIR-RECEIVER 161 

through it hurriedly, emerging at the other end practically 
as hot as when it entered, giving up its heat more or less rapidly 
to the inner surfaces of the pipes through which it travels, for 
which cooling effect the pipes and not the receiver should have 
the credit. 

The Receiver does not dry the Air. — While the air-receiver 
is not, thus, a cooler of the air, it also is not, and for the same 
reason, an abstractor of the moisture in the air, this being the 
most desirable service which it could render, and which it has 
been too readily assumed to do. It is well enough understood 
that atmospheric air — free air — always carries moisture and 
also always has capacity, or as we might say appetite for more, 
up to the point of saturation, when its avidity suddenly ceases. 
The moisture-carrying capacity of the air rises very rapidly with 
its rise in temperature, and diminishes, but not so rapidly, 
with rise of pressure. As the pressure must always be at the 
highest point just when the air is leaving the compressor, if we 
can then reduce its temperature to the lowest point, the air 
will be in a condition to surrender so much of its moisture that 
none will be found later to cause trouble, when lower pressures 
and perhaps higher temperatures are reached further along the 
line. It being thus sufficiently evident that the best place for 
drying the air is as near the compressor as possible, the first 
requisite is an efficient cooler, or rather after cooler, for the air. 

The air-receiver, as has been said, has never been a cooler 
of the air, and the builders are now testifying to this fact by the 
number of aftercoolers they are offering. The best of these are 
highly efficient, and the air after going direct from the compressor 
and through the intercoolers is in the precise condition desired 
— that is, of high pressure and low temperature — for the sur- 
render of its moisture. For the important service which the 
aftercooler may render, nothing could be expected to work 
more cheaply. There is only the first cost of the apparatus 
and connections, and then a sufficient supply — not large — of 
cool, free-running water. 

Now, surely, close to the aftercooler, the air-receiver should 
be available and effective for abstracting the excess of moisture 
which the aftercooler has liberated and for passing the air along 
so dry that there will be no trouble from moisture in the air any- 
where along the line after that. Unfortunately the typical air- 
receiver still persists in its inefficiency, and the air passes through 



162 



COMPRESSED AIR PRACTICE 



and out of it wet or carrying a considerable amount of con- 
densed but not separated moisture. 

It is to be remembered that when the saturation point is lower- 
ed by the lowering of the temperature of the air, and there is 
a relinquishing of the surplus moisture by condensation into 
water, that water still remains in the air, as mist or fog, and what 
is then needed is a separator. Any of the separators which are 
successful in drying steam are equallly efficient in taking the 
liberated water out of the air, when it is in the condition here 
spoken of. The efficiency of such separators is due to the habit 
which water has, and which liquids like alcohol, benzine, etc.. 




Fig. 49. — Draining the Water from the Line. 

do not possess, of wetting or clinging to the surfaces with which 
it comes in contact. A constant repetition of this wetting 
process causes the water to drip or flow off and accumulate in 
pockets provided, from which it may at intervals be drawn off. 
Draining the Pipe Line. — Fig. 49 tells its own story of what to 
do when there is a low spot in a long pipe line. There was in- 
serted here a short plain cylinder with the line pipe entering it 
at one end and leaving it at the other end close to the top, the 
bottom of the cylinder forming a water pocket from which the 
water was drawn at frequent intervals in considerable quantities, 
and there was no freezing up of drills on that line. 



I 



THE AIR-RECEIVER 163 

Improve the Air-receiver. — An air-receiver of the common 
type, preferably horizontal, if merely provided with a series of 
baffle-plates (which might be added without much additional 
cost) would be an efficient separator, provided that the air pass- 
ing through it were in the prescribed condition of high pressure 
and low temperature. Indeed it would seem to be quite possible 
to combine in one apparatus the two functions of cooling the 
air and separating the freed water from it, and it might also be 
made large enough to constitute a storage capacity equal to that 
of the plain air-receiver. It would seem to be almost an absurd- 
ity to be still installing the latter alone. We may look for 
some enterprising manufacturer to be putting on the market a 
combination air-receiver covering the three separate functions 
of aftercooler, separator and air holder. Since all these func- 
tions must be provided for in good practice, a single construction 
should be more compact, more efficient and cheaper than three 
separate devices. The writer has in mind one of the largest 
compressor installations in which each compressor unit has an 
aftercooler and then a receiver, with a separator added, after the 
completion of the installation, as a necessity. The separator 
should have immediately followed the aftercooler and the re- 
ceiver might perhaps as well have been dispensed with, as the air 
was delivered into long lines of large pipe. 

The intercooler or the aftercooler should be expected, or more 
properly required, to cool the air to within 10 degrees of the 
temperature of the cooling water. In 1906 Mr. H. V. Haight, 
of Sherbrooke, Canada, had the designirg of two air compressors 
each of 4000 cu. ft. per min. free air capacity, and in the con- 
tract it was specified that the temperature of the air passing 
from the low pressure to the high pressure cylinder should be 
reduced to within 15 degrees of the temperature cf the cooling 
water, with a penalty of $150 per degree in excess of this and a 
bonus of $150 for every degree below the 15. In actual running, 
at the required speed and pressure, the temperature of the air 
was reduced to within 5 degrees of that of the cooling water, and 
a bonus of $1500 was earned for the builders. 

There are indications that the air-receiver is soon to be doing 
better things. That it will be enabled to make good in all the 
particulars in which it is now deficient is too much, perhaps, to 
expect, but in what has been its principal accredited function, 
that of air storage, there is a notable recent development. 



164 COMPRESSED AIR PRACTICE 

Compressed-air Storage. — It may be said that compressed 
air wherever employed is always used more or less intermit- 
tently, and never at any constant rate, except in cases where the 
entire output of a compressor or of an entire compressing plant, 
is employed in a single water-pumping operation, so that the 
desirability or the necessity of air storage is not to be ignored. 
The air-receiver as generally installed not only does not hold 
enough to be of much account, but it does not maintain a con- 
stant pressure for a moment if the intake and the output vary, 
and so we look to the compressor for help. To insure somewhat 
reliable maintenance of pressure and volume it is the practice 
to provide a maximum compressing capacity somewhat in ex- 
cess of the maximum demand, and then to automatically reduce 
the speed of the compressor as the rate of consumption dimin- 
ishes. Even this arrangement usually does not completely 
satisfy the fluctuating requirements, and so we have various 
unloading or choking contrivances which will still more reduce 
the output without actually stopping the machine. However 
satisfactory the results thus obtained may be, it is evident that 
they are secured only by more or less complication of apparatus 
and a sacrifice of the essential conditions of power economy In 
the running of the machine. 

A magnificent opportunitj^ for the solution of this air-power 
storage problem opened to the engineers of the Anaconda Copper 
Mining Company, at Butte, Mont., when it was proposed to 
find a cheaper means of driving their great mine hoists than by 
the use of steam. There were at Butte 25 large steam-driven 
hoists with an aggregate capacity of 40,000 h.p., but the service 
required of the hoists was so intermittent, and the actual time 
of working of each was so short, that it was estimated that 
4000 h.p. in constant operation would be sufficient for all the re- 
quirements, but it was imperative that the power should be 
always ready and always sufficient for each individual hoist. 

The cost of steam had been about $80 per horse-power per 
year, while electric horse-power per year could be had for about 
$25. There were, however, serious objections to the adoption of 
the electric drive for each separate hoist, besides the enormous 
first cost of such an installation. With electric drive, also, 
there could be no power storage, so that it would be necessary 
at times to have current available for nearly all the hoists at 
once. 



THE AIR-RECEIVER 165 

So far as the steam hoisting engines were concerned, they 
could be adapted to the using of compressed air at comparatively 
slight cost, if only the power-storage problem could be solved, so 
that a constant drive of sufficient average capacity could be 
made able to take care of the peak loads whenever they should 
occur, even up to the running of all the hoists at once. The prob- 
lem has been solved with a success and completeness seldom 
surpassed in great engineering undertakings. 

It is not intended here to give more than the briefest sketch 
of the compressor installation and operation, the purpose being 
only to call attention to the air-storage scheme. 

The electric current which drives the compressors is trans- 
mitted 130 miles from the new plant at the Great Falls Water 
Power and Townsite Company, at Rainbow falls, just below the 
Great falls on the Missouri river. There are three compressors, 
each with a direct-connected Westinghouse motor of 1500 
maximum horse-power. The compressors, furnished by the 
Nordberg Manufacturing Company, are two-stage machines of 
the highest class, with low-pressure cyHnders 50 in. in diameter 
and high-pressure cylinders 30 in. in diameter, and a common 
stroke of 48 in. The combined free-air capacity of the three com- 
pressors I would estimate roughly at 20,000 cu. ft. per minute 
(not knowing the builder's specifications as to speed, etc.). 

From the compressors the air passes to the battery of air 
receivers Fig. 50. These are vertical, each 10 ft. in diameter 
and 30 ft. high, their combined cubical content being, say, 
70,000 cu. ft., which, at 90 lb. gage pressure may be said to 
equal 500,000 cu. ft. of free air, a volume which it would take the 
combined compressors nearly half an hour to compress and deliver. 
This is very different, to begin with, from the less than one 
minute capacity of the air receiver usually provided. 

But there is a more important feature and a greater difference 
and advantage to be noted in the present installation as com- 
pared with long-established air-receiver practice. The too 
familiar experience is that as soon as any air is withdrawn from 
the receiver in excess of what the compressor is delivering, or 
if for any reason the compressor stops, the pressure in the receiver 
falls rapidly and constantly with the drawing of the air. Under 
the arrangement here being considered, when the quantity of 
air contained in the receivers is diminished by any air consump- 
tion exceeding the delivery, instead of a drop of pressure rendering 



166 



COMPRESSED AIR PRACTICE 



the remaining contents of the receiver ineffective and useless, 
the pressure is maintained and the entire contents of the whole 
battery of receivers, including the original inert filling of air at 
atmospheric pressure, can be used at full pressure and effective- 
ness until the receivers are emptied. In practice the withdrawal 
of the air never goes as far as this. As these compressors run 
all day and all night, when there is at any time a simultaneous 
call for operating an unusual number of hoists, there is always 
the full capacity of the working compressors and also the entire 




Fig. 50. — Constant Pressure Air Storage — Anaconda Copper Mining Com- 
pany, Butte, Montana. 



contents of the battery of receivers to draw from until the unusual 
and excessive demand for air ceases. When there is such an 
unusual simultaneity of hoisting it is necessarily succeeded by 
a period when the hoisting and the demand for air are less than 
the compressor output, and then the receivers are automatically 
filled again. 

How the Air-pressure is Maintained. — The device by which 
the air-pressure is maintained in the receivers notwithstanding the 
diminution of the contained volume of air is essentially a simple one. 
It is accomplished by the use of a standpipe or its equivalent, the 



THE AIR-RECEIVER 



167 




Water 



same as in waterworks service. On a side hill at an elevation 
sufficient to give the required gage pressure of 90 lb., there is an 
open water tank 100 ft. in diameter and 15 ft. deep. A depth 
of 10 ft. in this tank gives a water capacity somewhat greater 
than the total cubic capacity of the battery of air-receivers. 
As 2.3 ft. of water gives 1 lb. pressure, the mean elevation 
of the tank above the receivers should be 90X2.3 = 207 ft. 
There is a large pipe connection from the bottom of this 
tank to a horizontal pipe in free com- 
munication with the lower ends of all 
the receivers. No valves of any kind 
are required, and httle, if anything, 
need be allowed for the friction of the 
water in the pipes, it being free to flow 
in either direction, according to the 
changes of the volume of air in the re- 
ceivers. No safety valves are re- 
quired, and it is impossible to pro- 
duce any pressure in the receivers 
greater than that due to the head of 
water. 

The compressed air as it is delivered 
from the operating compressors does 
not pass through the receivers, and, 
indeed, does not enter them at all ex- 
cept when the air production is greater 
than the consumption at the moment, 
when the surplus passes into the re- 
ceivers, driving out and up into the 
elevated tank some of the water at 

the bottom of the receivers. When the call for air is greater than 
the compressor supply then the deficiency is made up by a flow of 
air from the receivers, the water from the tank displacing it. 

The contact of the air with the water does not make it any 
wetter, as after compression it is quite certain to be saturated 
with water in any case. In the service for which this air is 
used, there is no call for "dry" air, as special means are provided 
for heating the air before it enters the hoisting engines, and 
moisture would be an advantage rather than otherwise. 

Long- continued records of the hoists as they had been run by 
steam made it possible to compute the compressor capacity 




M7 



Fig. 



51. — Constant Pressure 
Air Receiver. 



168 COMPRESSED AIR PRACTICE 

required, and to adapt the compressors to the work so that they 
could be operated at the point of best economy. 

The plant is unique as it stands, but in the use of the elevated 
tank it sets an example which, since there can be no monopoliz- 
ing of the principle, we may expect will in time be widely adopted. 
For the maintenance of a constant air-pressure with considerable 
storage capacity it seems to recognize and fill a long persisting 
requirement. Fig. 51 may serve as a reminder of the principle 
of it. The elevations which will give gage pressures of 50, 60, 
70, 80, 90 and 100 lb. are, respectively, 115, 138, 161, 184, 207 
and 230 ft. These heights can, of course, be secured as well 
by sinking the receivers as by elevating the tanks, or by a 
combination of both until the vertical difference is secured. 

It is to be noted that no water is consumed or wasted in the 
operation; it simply flows back and forth out of the receivers 
and in again as the volume of air in storage increases or dimin- 
ishes. Why not then dispense with the special elevated water 
tank and connect direct to a city or other water service where 
sufficient pressure is maintained, as it usually is? If the water 
pressure is constant and is somewhat greater or less than the 
air-pressure desired the latter might be adjusted by placing the 
air-receiver above or below what would otherwise be its normal 
position. 



i 



CHAPTER XVI 
PIPE TRANSMISSION 

In computations having to do with the profitable transmis- 
sion of air from the compressor to the work there are several 
particulars to be considered, such as the initial pressure of the 
air, the volume to be transmitted per unit of time, say 1 
minute, the distance to be traversed and the drop of pressure 
that may be permitted, the latter particular depending upon 
the size of pipe; and computations in air transmission usually 
are for the purpose of determining the pipe size. 

The ultimate decision as to the size of pipe in a given case 
is often in the nature of a compromise such as prevails in de- 
termining most business conditions. The pipe may be so 
large that there will be no appreciable drop of pressure, but 
this the cost of piping will prohibit; and then if it is sought to 
save too much in pipe cost by making the pipe too small 
there will be a constant loss in effective working pressure which 
will overbalance the saving. For the adjustment of the com- 
promise between the diameter of the pipe and the speed of the 
transmission experience is the most reliable arbiter, and in 
practice limits have been found within which those who work 
most profitably confine themselves. 

In considering the operation of compressing the air we base 
our computations upon the volume of free air taken in and com- 
pressed, but in questions relating to the transmission of the 
air after compression it is necessary to consider the actual volume 
of the air during the transmission, or usually either at the 
beginning or at the termination of the transmission. As the 
air soon attains the temperature of the pipe and its surroundings 
its temperature need not generally be taken into account as 
affecting the volume. 

The volume of free air transmitted ( figuring backward ) 
may be assumed to be directly as the absolute pressure, or as 
the number of atmospheres to which the air has been compressed. 
Thus, if we have a known volume of compressed air flowing 

169 



170 



COMPRESSED AIR PRACTICE 



Pi 






lO 


o 


>o 


o 


o 


o 


o 


o 


o 


o 


o 


O 


o 


o 


o 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 




^ 


00 


l^ 


kO 


■* 


(N 


,_, 


cr> 


00 


CO 


»o 


CM 


rr> 


r^ 


CO 


CO 


(-) 


r^ 


■^ 


CM 


(-, 


00 


iO 


CM 


OS 


r>. 


en 


CO 


o 


w 




00 


r^ 


CO 


lO 


■^ 


CO 




o 


05 


00 




CO 


CM 




o; 


r- 








o 


r^ 






o 




00 


CO 




g 






CO 


lO 


J^ 


05 




CO 


lO 


CO 


00 






00 


n 


CO 






iC 


1^ 




CM 














»c 
















""^ 










CM 


CM 


CM 


CO 


CO 


CO 


i^^ 


•^ 


■* 


■* 


IC 


»c 


CO 


CO 


CO 


CO 


t^ 


t>. 


s 




05 


00 


t> 


o 


o 


o 


o 


o 


o 


o 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


-aj 






o 


^ 


<N 


CO 


Tt4 


IC 


CO 


r^ 


00 


Oi 


^ 


CO 


CO 




CO 


00 


a> 


^ 


CM 


CO 


IC 


r^ 


00 


o 


^ 


CM 


■* 


CO 






O 


CO 


CO 


Ol 


(N 




00 




■^ 


r^ 


o 


r- 


CO 


CD 




lO 




t- 


'^ 


r^ 


o 


CO 


CM 


00 


IC 


00 




t^ 




P 




(N 






CO 






r^ 


05 






CO 


lO 


<X) 


(-15 




CO 


CO 


(X) 




CM 




CO 


OS 




•* 


>c 


h- 


OS 
























^ 










w 


CM 


CM 


CM 


CO 


CO 


CO 


CO 


CO 


■* 


-St* 


•<*< 


TJH 


rf< 


«o 






t^ 
t^ 


>o 


CO 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


O 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


O 




CO 


CO 


N. 


^ 


IC 


o> 


(N 


CO 


o 


■^ 


00 


lO 


CM 


CO 


o 


00 


lO 


CM 


o 


^ 


00 


CO 


CO 


(-, 


00 


CM 


IC 


CO 


o 


tf 






00 


CO 


lO 


lO 




O 




1^ 


lO 


CO 


o 


1^ 


lO 




o 








Oi 










^ 


CO 




00 


IC 










(N 


CO 


■* 


lO 




CO 


1^ 


00 


o 




CM 


CO 


»o 


CD 


a) 


o 


C-) 




CO 


lO 




OS 


C-5 




CO 


«f< 




























rH 
















CM 


CM 


CM 


(N 


CM 


CM 


CO 


CO 


CO 


> 
































































(N 


lO 






















































o 








(N 


■* 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 






1> 


■* 


^ 


00 


»c 


CO 


C-5 


l> 


■<*< 


1-H 


>o 


en 


h- 


^ 


00 


CM 


CO 


^ 


00 


IC 


OS 


tH 


00 


CM 


OS 


CO 


rH 


to 








05 




00 


CO 






r^ 


<N 


b- 


CD 




o 


o 




Tt< 


CO 


00 




CM 








o 


Tt< 




OS 


00 


CO 












1-* 


(M 


(N 


M 


CO 


-^ 


■* 


»o 


CO 


t> 


00 


00 


a, 


o 






CM 


CO 


■* 


rH 


CO 


CD 


2 


l> 


00 


fi. 




CO 


lO 




















































! 


Ph 






1> 


■* 


(N 


o 


o 


o 


O 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 






(N 


iO 


OO 


^ 


CO 


CO 


05 


(N 


■* 


r^ 


CO 


00 


^ 


Tt* 


a> 


-^ 


o 


»c 


00 


^ 


CO 


CM 


t^ 


CO 


iC 


00 


'^ 


C3S 


s 






CO 


o 


O) 


CO 


CD 


05 




CO 


Oi 




05 


lO 


05 


CM 






CM 


00 




iC 




00 








t^ 


Tf* 




















(N 


(N 


IN 


CO 


CO 


-* 


•* 


lO 


o 


CO 


l> 


l> 


00 


00 


OS 


OS 


o 


- 






CM 


2 


ID 






s 


§ 


00 


00 


O 


o 


O 


O 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 






o 


^ 


<N 


CO 


■^ 


IC 


CO 


1^ 


00 


05 


^ 


CO 


Tf 


IC 


r^ 


05 


^ 


CM 


CO 


Tt< 


CD 


00 


o 


CM 


CO 


•* 


IC 


l> 






(N 




CO 


00 


O 


(N 




CO 


00 




•c 


05 




CO 


r^ 




CO 


o 










r^ 








OS 




1 






















(N 


c^ 


CM 


CO 


CO 


CO 


^ 


■* 


»c 


iC 


lO 


IC 


CD 


CD 


l> 


t> 


t^ 


l> 








no 


CO 




















































1 


>; 


o 




i> 


lO 


CO 


1-1 


O 


I> 


-<:t< 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 






^ 


CO 


to 


t^ 


OS 


o 


(N 




CO 


00 


^ 


IC 


r^ 


05 


CM 


lO 


00 


CO 




»c 


o 


CO 


00 


o 


^ 


CO 


!^ 


rH 


tf 
H 


a 






(N 


CO 


■* 


lo 


t^ 


00 


05 


o 




•* 


CO 


r- 


00 




CO 


to 


00 


a> 


o 


CO 


»c 


r>. 


o 




CM 


1* 




<D 






















1—1 






r-l 


rH 




c^ 


CM 


CM 


CM 


CO 


CO 


00 


CO 


■"* 


-* 


Tt< 








00 


CO 






















































P-i 








CO 


lO 


t> 


o 


y-i 


(N 


•* 


o 


o 


o 


o 


O 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


O 1 


H 


a 


lO 


00 


CO 




(N 


,_, 


Oi 


1^ 


»o 


CO 


(N 


00 




CM 


,_! 


1^ 




o 


CD 






OS 


»c 


CM 


00 


CO 


tH 


rH 


•^ 1 


•« 






i-H 


O) 


CO 


■* 


■<* 


lO 


CO 


r^ 


00 


en 




CM 


CO 


^ 


CD 


00 


en 


o 




CM 


Tt< 


CD 


h- 


00 


OS 






W 


(4 


























I— 1 














CM 


CM 


CM 


CM 


CM 


CM 


CM 


CM 






M 




•^ 


r^ 






















































u 


-S 




N 


■^ 


t^ 


05 


(N 


'^ 


CO 


05 


o 


o 


o 


o 


O 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


<=> 




c3 

s 


'^ 


lO 


o 


to 


o 


CD 


,_, 


CO 


,_, 


t^ 


(N 


CO 


CO 


on 


•^ 


Tf 


lO 


lO 


iC 


,_! 


CD 


CO 


r>. 


00 


00 


CO 


00 


00 


OS 1 


O 










(N 


(N 


CO 


CO 


■* 


•<* 


lO 


CO 


t> 


l> 


00 


05 


o 




CM 


CO 


CO 


Tt< 


iC 


CD 


l> 


00 




2 


(N 


PO' 




8 


(N 


(N 


CO 




lO 


CO 


CO 


r^ 


05 






































In 




O 


o 


o 


O 


o 


o 


o 


o 


o 


1—1 


rH 


1—1 


rH 


l-< 


CM 


CM 


CM 


C<l 


CM 


CM 


CO 


CO 


CO 


CO 


00 


CO 


CO 


r-) 






■* 


00 


IM 


CO 


O 


Tt< 


00 


(N 


CO 


o 


00 


CO 


o 


Tt< 


CM 


o 


00 


CD 


o 


Tfl 


CM 


o 


00 


CO 


o 


■* 


CM 


o 
















<N 


(N 


CM 


CO 


CO 


■* 


Tt^ 


lO 


CO 


CD 


r>. 


00 


00 


a> 


o 


o 




CM 


CM 


CO 


Tt< 


1* 


IC 


CO 


^ 














































1— 1 


















IC 






00 


















































HH . 






05 


00 


00 


t>. 


t^ 


J> 


CO 


«o 


IC 


lO 


CO 


CO 


CM 


CM 


rH 


o 


05 


00 


00 


J> 


CD 


IC 


■* 


CO 


CM 


CM 


rH 


o 


Q 




eo 


(N 


lO 


00 


^ 




1^ 


o 


CO 


CO 


Oi 


lO 


^ 




t^ 


CO 


05 




o 


CO 


CO 


CM 


00 


rt< 


o 


CO 


CO 


CM 


00 












Tt 






(N 


(N 


IN 


<N 


CO 


■* 


1* 


Tt< 


lO 


lO 


CO 


1^ 


N. 


N. 


00 


00 


OS 


o 


o 


o 






;^ 




















































r-t 






r-^ 


T-t 


H 
H 




U5 


Oi 


CO 


00 


(N 


t^ 


rH 


CO 






■* 






































<N 


o 


o 




"—I 


IN 


(N 


CO 


CO 


-* 


Tj^ 


>o 


CO 


l> 


1> 


00 


05 


o 


r-t 


y-< 


CM 


CM 


CO 


■* 


tC 


U3 


CO 


t^ 


00 


02 
I? 




(N 


Tt< 


CO 


00 


O 


(N 


rf< 


CO 


00 


o 


■"i* 


00 


o 


CM 


CO 


o 


IC 


O) 


1-1 


CO 


r^ 


^ 


»c 


OS 


^ 


CO 


t^ 


r-i ! 

















1-1 


1—1 


1—1 


1—1 


CM 


CM 


CM 


CO 


CO 


CO 


1* 


•* 


■^ 


IC 


lO 


IC 


CO 


CD 


CO 


t^ 


t>. 


!>• 


00 


^ 


(N 


CO 


tH 


Tf< 


lO 


CO 




00 








































< 






CO 


CO 


05 


(N 


lO 


00 




»o 


1> 


1-f 


t^ 


CO 


CO 


05 


»o 


CM 


00 


"*< 


t* 


o 


CO 


CO 


OS 


IC 


00 


<-< 


» 


1*< 


tf 






^ 


(N 


CO 


lO 


CO 


l> 


Oi 


o 


^ 


CO 


lO 


00 


o 


o 


CO 


CD 


00 


^ 


CM 


■^ 


CO 


OS 


^ 




»c 


t^ 


OS 


CM 


H 























^ 










CM 


c^ 


CM 


CM 


CO 


CO 


CO 


CO 


CO 


rt< 


I* 


■* 


Tj< 


■<*< 


IC 


tf 


00 


CO 


■^ 


(M 






















































rt' 


CJ5 


Tt* 


a> 


-^ 














t>. 


CM 




CO 




iC 


IC 




IC 


CM 


•^ 


■^ 


CO 




CO 


CM 




H-l 

< 




rtIC 


00 


CO 


IC 


CO 


(N 


o 


a> 


1> 


CO 


T»< 




00 


t> 


lO 


CM 


05 


CO 


CO 


CM 


o 


t^ 


•<* 




00 


CO 


U5 


CM 


OS 




'"' 


O 


,_! 


(N 


CO 


TtH 


lO 


lO 


CO 


t^ 


00 


o 


,_! 


CM 


CO 


kO 


CD 


00 


o 


,_, 


CM 


CO 


«o 


r>. 


00 


OS 


o 


CM 


CO 






























7-i 














CM 


CM 


CM 


CM 


CM 


CM 


CM 


CO 


CO 


CO 


\^ 








CO 


(Ti 


(N 


















































n 






(N 


Tt^ 


CD 


05 


1—1 


■* 


CO 


00 


rH 


CO 


00 


CM 


>o 




rH 


CD 




IC 


CO 


00 




OS 


•>* 


00 


o 


CO 


r^ 


CM 






w|-« 


O 


(N 


00 


■* 


"-1 


t^ 


CO 


05 


CO 




-* 


t^ 


CO 


05 


CM 


■* 


t^ 


a> 


IC 




■* 


CO 


a> 




00 


Tt< 


CO 


OS 


[t\ 




'"' 


o 


,_, 


,_, 


oq 


CO 


CO 


Tt< 


rt< 


lO 


CO 


r^ 


00 


C5 


OS 


,_, 


CM 


CO 


■^ 


IC 


en 


r^ 


00 


OS 


,_, 


,_, 


CM 


CO 


Tt< 


§ 



































1—1 


















c>< 


CM 


CM 


CM 


e5 


!>. 




(M 




















































n 






(N 


lO 


00 


r-4 


-* 


CO 


03 


(N 


lO 


t^ 


CO 


00 


,-1 


CO 


Oi 


TtH 


o> 


»c 


00 




CO 




CO 


rH 


■* 


r^ 


CM 


00 


fj 






CO 


CO 


05 


CO 


CO 


05 


(N 


CO 


Oi 


CM 


05 


lO 


05 


CM 


00 


lO 




00 




IC 




00 


I* 


r^ 


■^ 


t- 


■<*< 


o 


o 






o 


o 


o 


^ 


1-1 


^ 


(N 


(N 


(N 


CO 


CO 


•* 


■* 


lO 


IC 


CO 


t^ 


t^ 


00 


00 


OS 


OS 


o 


m 


Hi 


:: 


CM 


CO 


I 




?| 


o 


o 


o 


o 


^ 


o 


O 


O 


O 


^ 


O 


o 


^ 


o 


<-) 


^ 


r> 


r> 


^ 


^ 


^ 


^ 


(-) 


^ 


o 


o 


^ 


o 


t-^ 


VH 


«o 




00 




o 




<N 




Tfl 


o 






o 


CO 




o 




Tt< 


o 


cc 


Ofl 


o 


c^ 


Tfl 


o 


CO 


00 


o 


> 
X 




II 








(N 




CO 


Tt* 


■>* 


lO 




t^ 


00 




OS 


o 




CO 












en 












15 
































rH 






" 


^ 


rH 


<-! 




^ 


CM 


CM 


(N 




m 


^'^'S 
























































' 


t-i 


■dj 


a o 


j ''^ 


IN 


CO 




IC 


CO 


t^ 


00 


05 


o 


CM 


•^ 


la 


CO 


00 


O 


CM 




»c 


CO 


00 


o 


CI 




»c 


CO 


00 


o 


m 


> 




























r-t 




CM 


CM 


c^ 


C^J 


CM 


CM 


CO 


CO 


CO 


CO 


CO 


CO 


1*< 


< 




S S^ 


























































H 




fe 



























































PIPE TRANSMISSION 171 

through a pipe at a pressure at 75 lb., gage, or say 6 atmospheres, 
the actual volume of free air will be six times as much. 

For computing or comparing cases of air transmission the 
linear velocity of flow in the pipe is generally adopted and is 
the more convenient form of statement. Table XVII gives 
the actual volume per minute of air passing through a pipe 
of given diameter when the linear velocity of flow is known. 
This is merely a convertible table of pipe capacities and will be 
useful as such in determining the size of pipe required for a 
given service. 

It is generally considered that for economical transmission 
the actual velocity in main pipes should not exceed 20 ft. per 
second, or 1200 ft. per minute, the latter quantity being more 
convienient as the minute is the time unit which generally 
prevails in compressed-air practice. 

It would be well if more attention were given to the capacities 
of the branch or distributing pipes employed. Why should 
there ever be higher velocities of flow here than in the mains? 
In actual practice it often occurs that, while the main pipe is 
large enough for the transmission, the smaller pipes or those 
through which the air is finally carried to the individual machines 
are too small, and velocities as high as 3000 ft. per minute or 
more are not infrequent. 

Even when the aggregate sectional area of the branch pipes 
is equal to that of the main which supplies them, so that the 
velocity of the flow should be the same as in the main, it is found 
in practice that to maintain that flow in the smaller pipes en- 
tails a more rapid drop of pressure, this drop of pressure increas- 
ing the volume and thus still further retarding the flow. 

The inadequacy of the small pipes of equal aggregate sectional 
area with the large pipe is due chiefly to the greater pipe surface 
and the additional friction caused by it. The interior surface 
of four pipes, 1 in. in diameter, for instance, is equal to that of one 
pipe 4 in. in diameter, but to have a pipe area equal to that of 
the 4-in. pipe we must have 16 pipes 1 in. in diameter, with con- 
sequently four times as much pipe surface for the air to rub 
against, as we might say. 

It is found that the diameters of branch pipes to equal that of 
the mains which supply them, or vice versa, should be as the square 
root of the fifth power of the several diameters, and upon this 
basis Table XVIII has been computed. The diameters of the 



172 



COMPRESSED AIR PRACTICE 





a 

s 

1 


CO 


^ 










c3 


W 




00 rl 




CS 






(N 


^ 


^ ^ in 




eS 


r-l «0 «0 
(N M "3 




s 


^ 


»C IC CO 

<o ec rM ^ 


w 
o 

< 


03 


«D (N «) 05 

i-H fO >0 00 


00 


XI 


kO CO rH rH O 




c3 


t> t> CD 00 U5 

.-1 (m' lO OS »c 


03 


t^ »0 (N (N 

CO I> 00 CO rH 

r^ t^ t>. 00 »o CO 

■^ (M rH O O O 


P5 


I-H CO CO CD 

^' ^ "^ d § ?? 


»o 


^ 


^"^ ^ ?5 S 2 

(M rH |> T-l lO C3 C<1 
CD CO rH tH O O O 


^ 


03 


CO (N CD Oi 

• • • • 00 (N O 
"-I CO »0 00 i-H CO lO 


o 


CO 
CO 


Xi 


X! 
e3 

c3 


lO lO C^ 00 00 
OOi-HOOINTti'-lt^'-l 
OOCOb-OCDCOrH^ 
lOCOi-Hi-HOOOO 


l>. t>. CO 00 .^ 
i-HINiOOSkOINCDOO 

rH CO >0 00 


u 

< 

<1 


OSCOCO00(Nt^OlO400 

CO^-ICOC^^t^TftC^r-lO 
t^Tt<C^rHOOOOO 


o 


CO ■'*< 00 00 00 

r^(N'cOI>CO.-<Ti<OOC^ 
T-H (N •* t^ -I 


> 


t- IC C^ C^ -^ t>. IC 

T**COI>00CD ^ICOOIO 
rH|~-t^t^OO>OCO»-IOO 
t^r)<(Nr-(000000 


h-3 




■>* 1-H CO CO CO 


1. 


(N 


c3 


lC-*(N00C^COi-l CT>»OC0 
INCO^t^'-HiCCOC^OOO 
COTjtcO-HT-lOOOOOO 


cococ^coo) cot-hcd 

00 (N O O 00 00 

rH(NC0lC00.-HC0lOrH^(N 




r-t 


X 


" — lO lO (N 00 CO lO .-1 

oO'-H ooc^-*'-ir^'r-(iococ<i 

S ^ ^ J::; 2 § § S S 8 § S 




c3 


'^^ .^^"^ ^CDOO 
i-H(N-*»OO5»O<NCO0000^OS 

I-H CO IC 00 rH to Tt* 




X) 
e3 


^^^^§S§S88§S 










^^.^cococo ^^„^ 
NcoicoorHococoi-ioor^io 

.-HC<)C0COrHrtf0CO 






- 


03 


»0 Tt< CSl 00 CO lO .-H 
i-HOOlM-^CO-Ht^i-liOCOC^ 

COt^OCO-^COrH^OOO 

eo^<-iOOOOOOOO 


















t^ CO 00 lO rH CD 00 
(NlCOSlOCOC^CDOOOOrHOS 

r-((Meo>oooT-HcoTt< 
















^(NC?COCOTj<iOCOOOO(NCOOTt< 

^ rH ,-( C^ (N 1 




'snTBui JO SJa^amBiQ 






i 



PIPE TRANSMISSION 173 

larger pipes or mains are given in the first vertical column at 
the left, and the numbers of small or branch pipes required to 
equal the carrying capacities of the mains are given in the suc- 
ceeding columns under the different diameters. The column a 
under each branch-pipe size gives the number of the smaller 
pipes required, and in column h is the reciprocal of this number, 
which will often be convenient as a multiplier, it representing 
also the fraction of main pipe equivalent to one of the branch 
pipes of the given diameter. 

Referring to the table, we note that 32 pipes 1 in. in diameter 
are required to equal the carrying capacity of a 4-in. pipe, which 
is just double the ratio of the squares of the diameters 16:1. 
So also it takes 316 pipes 1 in. in diameter to equal a 10-in. pipe, or 
more than three times the ratio of the squares of the diameters. 

The popular impression has been that great power losses are 
unavoidable in the transmission of air through pipes except for 
very short distances, but the facts do not sustain this view. We 
cannot do or get anything for nothing and, of course, there is 
some diminution of available power in the transmission of com- 
pressed air, but unless the piping system is very bad and in- 
adequate the transmission losses are among the lesser ones which 
have to be considered, and not at all to be compared with the 
loss of power through the heating of the air in compression or 
through the loss of volume by the cooling of re-expansion when 
the air is finally put to use. It would perhaps be quite as truth- 
ful, and more in accord with business practice in other lines, if 
those things were thought of and spoken of as expenses rather 
than as losses. 

In line with the transmission of compressed air, and subject 
to the same laws, is the transmission of natural gas from the wells 
for long distances to towns and cities where it can be most prof- 
itably used. Artificial gas also, used so largely for illuminating, 
for heating and other industrial purposes, as spoken of elsewhere, 
is coming to be more and more conveyed at high and higher 
pressures, so that here also the transmission question is of 
importance. 

There are no formulas for air or gas transmission which can 
be said to be absolutely right, or which are to be implicitly relied 
upon regardless of all conditions. Formulas which may suffice 
for ordinary practice, and which within reasonable limits tell us 
what may be expected to be the loss of pressure for a flow reaching 

13 



174 COMPRESSED AIR PRACTICE 

a certain limited distance, would, if the flow was continued far 
enough, show a gain of pressure instead of a loss. 

Table XIX of loss of head during the flow of compressed air 
through line pipes was computed by what may be called the 
Rix-Johnson formula. It gives results which are approxi- 
mately correct in practice, and I have reason to believe that 
it is now used in the United States more generally than any other 
in practical compressed-air work. J. E. Johnson, Jr., gave 
Church's familiar formula in a simplified form in the American 
Machinist, July 27, 1899, which was, in substance, as follows: 

where 

F = Volume of free air in cubic feet per minute; 
L= Length of pipe in feet; 
Z) = Diameter of pipe in inches; 

K = A numerical constant, which Johnson fixed as 0.0006; 
R = Difference between the squares of the initial and terminal 
absolute pressures in pounds per square inch; that is, 

E. A. Rix, of San Francisco, uses Johnson's formula with 
0.0005 for the value of K. In my own practice I have found it 
more convenient to use the reciprocal of this value of K, that is, 
2000, and transfer it to the divisor, where in actual numerical 
operations it almost invariably disappears at once by cancel- 
lation. I also find it more convenient for my pencil habit to 
use small letters instead of capitals; hence my working formula 
takes the form. 



p^i—p^2^ 



v^l 



2000 d^ 



This, of course, may be transformed to suit the special require- 
ments, as: 



2000 d^ {V\ -V\) 



,.4 



d' = 



2000 W(p\-p\) 
" I 

vH 



2000 {p\-p\) 



PIPE TRANSMISSION 



175 



w N o 00 lo 

r-< »0 IH to «0 

d d cq Tj! 00 



^ a 



•2- S fl 
a ft. 5 






O O »-l (N ■* 



03 00 r* «o CO 

00 t^ lO CO i-H 

(N »C r-l t^ CO 

rH I-H (N 



»0 O O O O 



O) lO O lO 



d d 


IN 


lO 


d 




00 


o 


■^ 


o o 


^ 


■<* 


t^ 


O CO 


2 


(N 


28 


o o 


^ 


«N 


■* 



ft S 2 

•ft ».s 

a 



a " 



^ 9 SJ § 
o 'S. ft .s 

S a 



ft . 

^ |.2 ^ 

I "a " 



O >-" (N -^ lO 

CO «0 <N 00 ■* 

00 «0 CO o> to 

i-l i-H M 



>c o o o o 



O O <N CO 



CO 


CO 


;^ 


(N 

00 


CO 


o 


o 


<N 


xl* 


00 


§ 


^ 


5 


00 




o 


o 


^ 


CO 


»c 


° 


00 


S 


00 


00 



O O O ^ <N 



ifl .-I CO Tjt 

00 t^ ■* ^ « 1 

CO t- lO 00 O 

i-( d 00 I 



kO o o o o 

(N lO d lO Q 
.-I M U5 t- O 





4^ 

1 

ft 
■ft 

a 


fl 


t* 00 CO (N ■* 

<-i CO r^ 00 »o 

d d (N CO rH 






8 


O O C<l Tj< 00 






° ^ ^ -. ^ 
d d --H CO lo 






i 


d d d --< <N 








(N lO O S O 








<N lO O lO O 






"o 
a 


l-H 


O O 00 t^ oo 






§ 
t^ 


lO 05 00 CO t^ 
•-I »0 00 ■* « 

d d IN "5 d 






1 


2 g 2? g ^_ 

d d ^ 00 d 




ft 

ft 


g 

(N 


»0 0> 0> 00 00 
O '-I t» t^ "-I 

d d d r-! 00 


, 






<N IC rH t^ 00 


c 






^g§S| 






43 

i 

ft 

•o 


1 


d d eo 00 d 






S 


d d <N d j^ 






2 ^ S S3 S 

d d r-! TtJ t>^ 






C4 


§ s ^ § ?: 

d d d ci CO 








ggSJ;;2 










^gggg 







1 
.9 

i 
■ft 

t 

a 


1 


»-H ci »o 00 <N 




1 


d ^ IN ^ d| 




i 


<N Tt< (N OS CO 
00 t^ CO o o 

d d >-! IN CO 1 




i 


d d d d r-i 1 






1 

J2g^?§2| 

«0 00 rH TJ< 1> ] 






ggggg 
l-H rH (N M CO 

1 




43 
1 

d 

<u 
ft 

■ft 

1 

J 


I 


g 5, ^ t2 5;; 

^ 00 d d Tji 




S, g § J5 ^ 

d rH CO Tjl d 




g 


t^ 00 l-H t^ lO 

CO 00 »0 05 Tt< 

6 6 r^ ^ Oi 


1 

ft 


i 


d d d d r.; j 


(N 




CO 05 00 CD 0> j 

1 






1 

o o o o o 
O kO o »o o 




1 

a 

i 

ft 

*© 


1 


1.75 

3.99 

7.24 

11.60 

17.28 




t^ N- ■«** O 00 j 

00 CS "3 CO rH 
d r^ 00 d 00 




0.171 0.43 
0.39| 0.98 
0.69 1.75 
1.091 2.31 
1.57! 4.92 






-- 


J* CO 00 O CM 



II 



176 



COMPRESSED AIR PRACTICE 





io 




C^ 


a 












a 






o 




o 


a o 






p. 





1°; 

"S'^i 

fl VH Ot-KNIOOO 

(^■ — i ' 

lO !(MC0-*»C»O I 

I o i CO 00 a> •* Th I 

I |0; 

-^lOO^COW 



CO CO C O CO 
IcOt^'-iTPX 



§ ! CO ■>* CO (N t^ 

"SiJ2id<N»od«o 

•-^ o ; -^ « ic o CO 
0) : o ' lo ci •* o lo 
CO 

O , O ' 00 t^ ■<*< C 00 

_c O ; CO ■* CO O CO 

y; tH C-HCOCOO 

C I 

0) 

-H O|iO00(NXiO 
I O I !N C5 <N 05 (N 

Ol 

T-l O O (N CO CO 



lOsxt^coo 

ITtXNCO'H 



looooo 

lOOOOO 

:-i(Nco'*ic 



1 

i 

a 
•o 

J 


1000 ! 1500 1 2000 1 2500 i 
0729 1 0.44 0.59 i 0.73 
1.16 i 1.74 2.32 2.89 
2.6 3.91 5.25 6.64 
4.64 6.90 i 9.30 ■ 12.20 
7.50 10.00 15.10 18.90 


lOSXt^COO 
X t^ CC O lO 



.sr 



lo 

1-1 i"-" 



OOCOW'* 



COeO(NCO(N 
^iC.-iXI> 

d d (N -*' 00 



OOi-iC0»0 



■*05 05XN. 
uO O .-I iM CO 

jC^iOCOO 



lOOOOO 

ooooo 

lf-i(NTtCOX 





— 





. — 






o 


»ooso>-*a> 






(N OS OS -It*' 




^ 


C4 


OOCOOSCO 




d 


3 






ooos^eo 




a, 


§ (Nt^-HCO-* 

<^ ddcot>ieo 




& 




s. 


•o 


1 

O|iCCT>C5C0t^ 


p. 


j3 


g ,r-.lOC0-*X 


c 


hT 


^idd(Niod 










CO 


•^ o 


005XC5^- 






^COICIO-* 




_h 


OOrHCOCO 




.-HC0-*COX 






a)XcO'<*(N 






(NiO^t^CO 






rtrH(N 


















-^(N^COX 



§ (N^COX^-i 
l^ld^^dd 

•rt O CONlfllO^ 
gj O (NOt^CO»-l 

.2- <N ddcoo6cc> 
a r^ 

' o o r^050305-<* 
_ o ^cot>.cot^ 
-5 o 

■g, ^ OOINCO-^ 

13 -^ . n 

J O (NCOCO^Ti* 
O ^Tt^XNCO 

o 



ooxt^co 

COt^lCCO-H 

CO CO coot*, 



ooooo 
ooooo 

-i'NCO-<*lOl 



ooooo, 
ooooo 



1 

a 

<u 
a 
'a 
•o 

1 


CM 


d ^ m' "5 00 


0.26 
1.02 
2.31 
4.14 
6.53 


I 


OCOCOOiCD 

dd'-Hco'* 


0.12 
0.51 
1.15 
2.05 
3.22 


cot^o^t^ 


S8§§§ 

(NiCt^OIN 


49 

a 

"o 

1 


w 


drHcoiod 


0.29 
1.17 
2.65 
4.76 
7.53 


^ 


dd^coic 


lOX — 10 05, 

.-H1OC0C0-* 
d d 1-! ci CO 


! 1 

■*X(NCOO 


1 

'"a 
"o 

1.5 


§ 

CO 

§ 

o 

s 


C^iOt^OCd 

d ^' CO i>i ^ 


0.34 
1.37 
3.10 
5.60 
8.91 


0.17 1 0.26 
0.68 ! 1.02 
1.54 ! 2.32 
2.76 ! 4.17 
4.34 i 6.52 


t^iCNON. 

r>.ioco^x 

tJ<OStJ4OSC0 






lilii' 



Jo'OcOOiC^iO 

io.t^^^-:© 

'-i CMCOr-<o6t>^ 
.ti O , rH.-l(N 

«|-: ^ 

.Si tJ<tJ4C0 05»0 

O ■ CO O ■* CD lO 

4> O 

a o -H CO »o X •* 
"o 

JS O t^.-iOt>-CO 

*i O cOiOt*(NO 

M iC 

C C<l O'-lC<lrJ<C0 

^■-, — 

o ;t^i-Hxoseo 

'Ol(NCOOcOO 

iOl 

i i-i I O O --I '-I cq 



t* LO TjH .-I O 

comt^os^ , 

C0 05CM»fl05 ' 



ooooo 

OOOOO 
OiOOUOO 
'-^ 1-1 C<1 cq CO ' 





s 


2:2gs • 


V 


d 


cob-eO'-H 








"a 


S 


1.54 

3.51 

6.31 

10.07 

14.84 


'"c 


o 


t^r^i-HX^ 




CM 


C-<cO'*t>- 








i-H 


^COCOCOCi 
CO CO CM 05 t> 


_ 


OO-t-iCSI 



i iCMOSiC-HX 

I t^O-^X-H 







1000 
1500 
2000 
2500 
3000 


1 

c 

V 

a 
"a 

'o 

1 


o 

— 

CO 


iCCOrJ*-* 
:OTt<CO-H 




COXlOCO 

I-H CO 




X^^SS 




^i*t*-^N 


XCCOb-Tt 

dcocoiox 


O ^ ■* CO X 
COX-<i<CMC<l 


OO^NCC 



•oscoxcot* 

I T}< r» oi CM •* 

OOCMCO^lC 
i r-n-HeMCM ; 

■oooooi 

OOOOO: 

o>co»co 

-^1-tCMCMCO \ 



M 



PIPE TRANSMISSION 



177 ' 



it^ 



-H (N lO 00 CiJ 



■-I t^ CO Oi Tjt 
^.-^.-H(^^ 



■ a 

'a 

a i^ 

00 % 



OOt^COlO-* 

rH(NeO-<*lO 

00<NtOOrJ< 



ooooo 
ooooo 
ooooo 



o cot^t^^ 

O Tj<Oil^io 

O I • ■ • . 

" -KM 



O »OiMCOCO00 

»o • • • • • 
C^ ,O-HC0i0t^ 



O !^?^t^«50 

ol 



O d f-i csj CO 



«0«*SO00«) 
♦< OS f«5 00 



Mooeot'.i-* 
d d f<5 1>^ ^' j 



OOi-fCOCD 



lOO^rH^ 
COt^rJ(T-t00 
<N-*OStJ<00 



oofoooso 



OOrH-^t^ 



00b.U5<MO 

(NkOOCOrH 



2 oO(N<r)(Nir) 

O (MrHiOCOO 



O Or-lTj<0 



oocot*-*! 



00<NiC05 



O O .-I (N ■* 



CO f>» ^ ^-* 00 
"-IINICOOO 
CO«O(N00ifl 



>oooo 
>oooo 

)0000 
<(N'«»<«00 



Olco.-<-i(»00 
O|»0rtW^00 

O I d IN •* 00 « 



OOOCO—IO 
0> 05 05 05 00 



OOOOO 

ooooo I 
»oo »c o »o I 
e4">o i>o ei I 



o<Niooco; 



;0 lOr^OiOXD 

lo eo(Ncoooi> 
o 

"3 0.-H(NTj(t>. 



iOO>COW5 



§§§88 

«5 0>0 0»0 
M"iOt>o'(N 



g OONiOiOOS 
O ]CO00Tt<OJ»O 



OINCO^OS 



O'-ICCIOOS 



WOW'- - 
.-H^CSKN 



178 COMPRESSED AIR PRACTICE 

Computations in this line do nor invite, nor hardly permit, 
micrometrical precision, and refinements are out of place; hence 
it is quite permissible and very convenient to use 15 lb. for the 
normal atmospheric pressure, and this has been done in com- 
puting the tables herein given. A single example of the process 
will suffice. 

Let there be 5000 ft. of 8-in. pipe, through which it is desired 
to transmit 4000 cu. ft. of free air per minute at an initial 
pressure of 105 lb. gage, or 120 lb. absolute. What will be the 
terminal pressure and the loss of head? The pressure here 
assumed is not unusual in the best practice of the present day. 

Note that d^ (8^) is 32,768, and v\ (120^) is 14,400. Sub- 
stituting these values the statement and solution is as follows : 

Then, 

^2^ = 14,400-1220 = 13,180 

and 

P2 = V13180 = 114.80 lb. 

absolute terminal pressure; hence the loss of pressure is, 

120-114.80 = 5.20 lb. 

The foregoing example shows what may be considered as near 
the highest permissible rate of pipe transmission, or a flow which 
should not be much exceeded in practice. The free air in this 
case being 4000 cu. ft. per minute, and the initial absolute 
pressure being 8 atmospheres, the actual volume, assuming 
that aftercoolers are used and that the air is at normal tempera- 
ture, will be only 500 cu. ft. per minute. The volume content 
of an 8-in. pipe is 0.349 cu. ft. per foot of length; therefore the 
rate of flow will be 

500^0.349 = 1432 ft. 

per minute. A handy limit figure to keep in mind is the round 
number 1500 ft. per minute. 

The loss of pressure will be a little more than proportional to 
the squares of the volumes of free air. That is, if in this case the 
volume of free air had been doubled, making 8000 cu. ft. instead 



PIPE TRANSMISSION 179 

of 4000 cu. ft., the loss of head would have been 22.44 lb. instead 
of 5.20X4 = 20.80 lb. 

It is worth while to note how the pressure loss is diminished 
as the pressure is increased, due to the reduction of volume. 
Thus, in 1000 ft. of 1-in. pipe, transmitting 50 cu. ft. of free 
air per minute, the diminishing pressure losses for increasing 
initial pressures would be as follows : 

Initial pressures Loss of pressure 

45-lb. gage 11.52 lb. 

60-lb. gage 8 . 86 lb. 

75-lb. gage 7.24 lb. 

90-lb. gage 6. 14 lb. 

105-lb. gage 5.33 lb. 

120-lb. gage 4.71 lb. 

135-lb. gage 4.23 lb. 

150-lb. gage 3.83 lb. 

Of course, no tables can be compiled which will cover all the 
various requirements of compressed-air practice, but the figures 
herein given may at least furnish a working idea of the probabili- 
ties, and may be of service in a general way in preliminary esti- 
mates, or may serve to detect errors or inconsistencies which 
are apt to occur in the most careful figuring. No precise agree- 
ment with actual practice can be expected, as conditions which 
affect the result are so numerous. 

The condition of the pipe itself no formulas can make allowance 
for. The actual diameters of wrought-iron or steel pipe, especially 
in the smaller sizes, are different from the nominal diameters. 
Some pipe is smooth and some has seams and blisters. The 
pipe may be straight or it may have numerous crooks and some 
elbows. As the computations of pressure losses usually have to 
do with transmissions to considerable distances few elbows are 
likely to occur, so that they seldom have to be taken account of, 
and if we say that an elbow gives a resistance equal to an addi- 
tional length of pipe, we will probably come as near to it as any 
arbitrary formula could inform us. 

No table or formula can take into account foreign substances 
or obstructions in the pipe, and it should be unnecessary to advise 
blowing out the pipe before it is put to use. If pipes follow the 
irregularities of the ground there may be low places where 
water will accumulate and interfere with the free flow. Water 
if it cannot be kept out of the pipe should be got out, and these 



180 



COMPRESSED AIR PRACTICE 



low places offer the opportunity for draining, as spoken of 
elsewhere. 

Strength of Pipe. — There is generally little question as to 
the strength of standard piping for the air-pressures generally 
employed, say up to 8 atmospheres or 105 lb. gage, but for the 
higher pressures special piping may be required. A generally 
accepted formula for computing the bursting point of pipe of 
given thickness is : 

2IXS 

^~ D 

in which P = bursting pressure, gage 

7 = thickness, inches 

>S = tensile strength of material. 
What is the bursting point of 10-in. wrought pipe 0.366 in. 
thick, internal diameter 10.019 in., tensile strength of metal 
50,000 1b.? 

2 X0.366X500 00 _ _ _ ._ 
10.019 ~ -^^^'^• 

The above formula may be transformed to find the thickness 
of metal : 

PXD 



1 = 



2S 



What thickness of metal is required in a pipe 12 in. in diameter 
to burst at 3500 lb., tensile strength of metal 45,000 lb.? 

^ 3500X12 



2X45000 



= 0.466 in. 



The tensile strengths in the above formulas may be considered 
safe in practice, the larger one being for steel. 

Liberal factors of safety should be used, say 5 for air when 
there are no shocks or excessive temperatures to be encountered, 
but the factor when adopted should be held imperative. 

Crane & Compan}^, Chicago, did a good public service in 
testing some samples of pipe to the bursting point. 

10 in. standard wrought iron burst at 1900 lb., while the rule 
gave 2922 lb. 

10 in. extra strong wrought iron burst at 2700; by rule, 
4102 lb. 

None of the pipes burst at the weld, the rupture in each case 
being some distance from it. 



CHAPTER XVII 
RE-HEATING COMPRESSED AIR 

It is sufficiently well known that after the transmission of 
compressed air to the point where it is to be employed, it having 
lost its heat of compression on the way, or, indeed, having had 
that heat abstracted by aftercooling devices, a considerable 
saving in the cost of the available power may be effected, 
theoretically at least, by re-heating the air before it is put to 
do its work. We still, however, have little reliable practical 
data concerning this suggested economy, or actual knowledge 
of the precise conditions under which it becomes available. 

The specific heat of air being low, comparatively little heat 
is required from an external source to raise the temperature 
of compressed air rapidly and, under constant pressure, to in- 
crease its volume correspondingly. While air at a low tem- 
perature has a comparatively small cooling effect upon water, 
or upon whatever may come in contact with it, the fact inversely 
applied may be taken advantage of in the use of air as a power 
transmitter. 

It may easily be shown that where a certain volume of air has 
been compressed to any given pressure, and has, by aftercooling, 
by transmission or by storage, returned to approximately its 
normal temperature, if that air is then re-heated and thereby 
expanded, the additional volume of compressed air resulting is 
produced by a much lower expenditure of heat than the original 
volume of compressed air was produced for, and also by a much 
lower expenditure of heat than would be required to produce 
an equal working volume of steam. The actual figures in the 
case, all theoretical, are as follows : 

Weight of 1 cu. ft. of steam at 75 lb. gage = 0.2089 lb. 

Total units of heat in 1 lb. of steam at 75 lb. from water at 
60° = 1151. 

Total units of heat in 1 cu. ft. of steam at 75 lb. = 1151 X 
0.2089 = 240.44. 

To produce by compression through a steam-actuated air- 

181 



182 COMPRESSED AIR PRACTICE 

compressor 1 cu. ft. of compressed air at 75. lb and 60°, about 
2 cu. ft. of steam of the same pressure are required, or the heat- 
units employed in producing 1 cu. ft. of compressed air will 
be about 240.44X2 = 480.88 heat units as the thermal cost 
of 1 cu. ft. of compressed air at the above temperature and 
pressure. The temperature and volume of the air as it leaves 
the compressor will be considerably higher than the figures 
here assumed, but as the air is invariably stored for a time, 
or is transmitted through pipes to a distance between its com- 
pression and its ultimate employment, it may be said to always 
return to its normal temperature before it is used, so that, 
whatever we may have at the compressor, the air as it is delivered 
to the motor, or whatever apparatus may be operated by it, 
will have cost, as above stated, 480.88 heat units for 1 cu. ft. 
at 75 lb. The difference in the thermal cost of any volume 
of compressed air thus produced by mechanical compression 
and the cost of any additional volume of air that may result 
from the subsequent re-heating of the air is very striking. 

The weight of 1 cu. ft. of free air at 60° = 0.076 lb. 

Weight of 1 cu. ft. of compressed air at 75 lb. and 60° = 0.456. 

Units of heat required to double the volume of 1 lb. of air at 
60°= 123.84. 

Units of heat required to double the volume of 1 cu. ft. of 
compressed air at 75 lb. and 60°= 123.84X0.456 = 56.47. 

Cost of 1 cu. ft. of superheated compressed air at. 75 lb. com- 
pared with the cost of 1 cu. ft. of compressed air as produced 
by ordinary compression: 

480.88: 56.47 ::1:0.1174. 

Here we see that the cost in heat-units of the volume of air 
produced by the re-heating is less than one-eighth of the cost of 
the same volume produced by compression. Upon this showing 
the matter is certainly worth looking into. 

The operation of re-heating compressed air is correctly so 
termed. It is, in fact, a case of doing work over again, or of 
replacing in the air heat that has been lost by it in previous opera- 
tions. It must be confessed that the presumption is all against 
our finding much profit in this direction. There are .not many 
places in life where it pays to do our work a second time. There 
is, as we have seen, practically no air compression without heat- 
ing the air by the operation, and there is no transmission of air 



RE-HEATING COMPRESSED AIR 183 

after compression without its cooling to very near its original 
temperature. If the air could go immediately from the compress- 
ing cylinder into the motor cylinder, where it does its work, 
without losing any of its heat, it would have the same effective 
power as it would have after long-distance transmission and 
cooling and re-heating, and without the additional cost of that 
re-heating. 

While we are saying in all good faith that there is little loss 
of power in the transmission of compressed air to considerable 
distances, and that the difference in the pressure of the air at 
the two ends of a long pipe necessary to overcome the friction 
and maintain the flow is but small, and that it is to a great 
extent compensated for by the increased volume at delivery, 
the fact still is that there is a great loss of power in the transmis- 
sion of the air, if we reckon from the moment when compression 
ceases, on account of the inevitable cooling of the air. Still 
this loss is not properly chargeable to the transmission, for no 
matter how far the air may be transmitted the cooling is all ac- 
complished before the air has travelled very far, if the pipes 
are of proper size. Supposing air to be transmitted 10 miles, 
it must be conveyed with considerable rapidity if it does not get 
down to normal temperature before the end of the first quarter 
of a mile. 

As the volume of air under any constant pressure varies 
directly as the absolute temperature, it follows that to double 
the volume by heating the air its absolute temperature must 
be doubled. The air being at 60°, its absolute temperature will 
be 604-461 = 521, and double this will be 521X2 = 1042, the 
absolute temperature required. This by the thermometer will 
be 1042 — 461 = 581°. As this is the temperature that is re- 
quired for the air when delivered into the motor, and actually 
beginning its work, it will be necessary, on account of the ease 
and rapidity with which it cools, to heat the air considerably 
higher than this theoretical temperature. It is one thing, and 
an easy one, to heat the air, while it is a very different and a very 
difficult thing to keep it hot. To avoid all loss of heat it would be 
necessary, not only to keep the pipe which conveyed the air 
constantly hot, but also the cylinder in which it was used, or it 
would be cooled before it began to do its work. 

In one case within my experience, where compressed air was 
re-heated, and its absolute temperature was increased at the 



184 COMPRESSED AIR PRACTICE 

heater 38 per cent., and where, of course, its theoretical increase 
of volume was the same, the actual increase of power realized 
was only 12 per cent. In this case the air was transmitted after 
the re-heating about 20 ft., the pipe was not covered, and no pre- 
cautions were taken to prevent loss of heat by radiation. The 
volume of air transmitted was sufficient to develop between 20 
and 30 h.p. The theoretical augmentation of temperature re- 
quired to double the volume of compressed air at 60° being 
581°, the actual temperature required at the heater under the 
most favorable conditions in order to have a double volume of 
air available in the motor will not be less than 800°, and this is 
a temperature that it is practically impossible to employ and 
maintain, lubrication troubles would defeat it if nothing else, 
and we may as well give up all thought of doubling the volume 
of compressed air by re-heating it and of realizing the promised 
economy of such an operation. 

If instead of doubling the volume we only attempt to increase 
it by one-half, or 50 per cent., which it is practicable to do, the 
required theoretical temperature (absolute) will be 521 -f 50 per 
cent. = 782, and 782 — 461 = 321°, the sensible temperature re- 
quired. Adding enough to this to allow for the intermediate 
cooling, the actual temperature required should probably be 
not less than 450°. The temperature of the air before the re- 
heating being assumed to be 60°, the increase of temperature will 
be 450° -60° = 390°. As we saw above that it required 56.47 
heat units to raise the temperature of 1 cu. ft. of compressed 
air at 75 lb. gage pressure from 60° to 581°, the actual increase of 
temperature being 581 — 60 = 521, it follows that to raise the 
temperature 390° will require: 

521 : 390 : : 56.47 : 42.27. 

Then if the first cubic foot of compressed air costs 480.88 heat 
units for its compression, and if the additional half of a cubic 
foot produced by re-heating costs 42.27 heat units, the total cost 
of 1 1/2 cu. ft. under the re-heating system will be 480.884-42.27 = 
523.15, and the cost per cubic foot at this rate will be 523.1^ 
1 1/2 = 348.76 heat units. The relative cost in heat units of 1 cu. 
ft. of compressed air produced by compression alone, and of a 
cubic foot resulting from compression and re-heating, will be: 

480.88 : 348.76 : : 1 : 0.72. 



RE-HEATING COMPRESSED AIR 



185 



From this it appears that the gain by re-heating compressed 
air sufficiently to increase its effective volume 50 per cent, will 
be 28 per cent. The more fair and correct way to state this, 
however, will be to reverse it: 

0.72 : 1 : : 1 : 1.38. 

We may say, then, that the total fuel applied with the re- 
heating system will yield 38 per cent, higher results than are to 
be realized without the reheating. This seems to be very near 
the maximum that can be attained in the way of economy by 
re-heating dry compressed air. 

But, after all, it must be confessed that it is not always, nor in- 
deed often, that the re-heating of compressed air is practicable or 
possible. Bearing in mind the facility and rapidity with which 
heated air in transmission loses its heat, it is idle to think of ever 



461' Abs 


561 


" Temi 


. 661 = 


Fahi 




761' 










861" 








961^ 


1061^ 


U« 




























p 






^ 


■^pf 


TL-'"' IP'' 
































1 L 


V. 


^ 






LJ-' 


iX - ■■ ' \ - 
































-5v 


^V 






' 


■J-T 


Y^ _ ,''' 






•^ 




















\ 


^ 




■«^^ 




- 


Jr 




a\ 


lj3^ 


'"x^ 






5 














\o\ 




- 






."' 


X" ' 






ij 


fi^^L 
























,' 


1. 




' 




,-'■ 


,,- 


































,•' 


" 


'- 




' 






,''' 






























-■ 


-- 


' 




^, 


* 




' 


































::=:; 


^- 


-;;, 








' 




































ii ' ' ' 


y. 


_.- 












































a 




























L 























0° Temp. 100° Fahr. 200" 300° 400" 500" 600' 

Fig. 52. — Increase of Volume by Reheating. 



profitably re-heating compressed air except for continuously run- 
ning motors, and then by heaters very close to the motors. 

Wherever any motor or engine is to be run by compressed air 
without interruption, a heater for the air should certainly be 
employed; but there are now several hundred different and dis- 
tinct uses of compressed air in not one of which it would be prac- 
ticable or anything but a losing operation to try to heat the air. 

Fig. 52 shows the increase of volume accompanying the heating 
or reheating of compressed air. The air is assumed to be heated 
from the several initial temperatures of 0°, 32°, 60°, and 100°, 
the pressure remaining constant during the operation represented. 
The relative volume at any temperature is indicated by the height 



186 



COMPRESSED AIR PRACTICE 



of the vertical line corresponding with that temperature, the 
height from AB to CD representing one volume, and each 
horizontal line above that indicating, successively, an additional 
one-tenth of volume. When the line EF is reached, the original 
volume is doubled. Figures below the base-line AB indicate 
the sensible temperatures Fahrenheit, and the figures above the 
upper line indicate the corresponding absolute temperatures. 

Fig. 53 shows the increase of pressure only caused by the 
heating of compressed air, the volume being constant. The 
air is assumed to be heated, as in Fig. 52, from the several initial 
temperatures of 0°, 32°, 60°, and 100°, and also from a number 



Absolute Temperatui-e Fahrenheit 

661" 761° 861° 96f 



1161* 
165 




100 Temp. 200' Fahr. 300" 400° 

Fig. 53. — Increase of Pressure by Reheating. 



of different initial pressures. The pressures are indicated by 
the several horizontal lines, the vertical distance between any 
two adjacent lines representing approximately 1 atmosphere. 
The figures at the left of the diagram indicate the gage pressures, 
and the figures at the right the absolute pressures. The tem- 
peratures are indicated as in the previous diagram. 

It looks as if the most unsatisfactory phase of the re-heating 
problem to-day. is in the re-heater itself. There are re-heaters 
so-called of various makes on the market, but where is the 
re-heater? It is not enough to provide a receptacle where fuel 
may be burned and then sufficient heating surface over which, 



RE-HEATING COMPRESSED AIR 187 

or heated tubes through which, the air to be heated may be passed. 
This might do when the air was flowing steadily all day long, as 
for instance in driving a mine pump, but where else of all the 
places in which compressed air is used? 

Supposing the heater to be of the right capacity and the fire 
adjusted to give the required air temperature when flowing at a 
certain rate, then if the air flowed faster it would not be heated 
enough, and if it flowed slower it would be overheated. If the 
flow stopped entirely and then after a time started again the air 
might be so hot as to burn the packing or make other trouble in 
that way. In either case there would be waste of fuel and the 
economy sought would be only partially realized, if at all. 

Then with this arrangement who is to know what the actual 
temperature of the air is, except by watching a recording ther- 
mometer and then only to realize that the heat is not right and 
is not steady at any figure? The proper working temperature for 
the air should be decided upon before hand, and then the ideal 
reheater should maintain this temperature constant, within 
narrow limits, just as an intercooler or an aftercooler maintains 
an approximately constant temperature when working in the 
opposite direction. These coolers deliver the air at a temperature 
approximating that of the circulating water whose temperature 
varies only imperceptibly; so it would appear that if water were 
employed as the circulating medium, and if this water were 
kept at a constant temperature then the temperature of the air 
heated by it could be depended upon to be nearly constant. 

The temperature of the water in a steam boiler rises or falls 
with the steam- pressure, and if the steam- pressure remains 
constant the water temperature coincides with that of the steam 
— or rather vice versa. By carrying a proper steam-pressure the " 
right water temperature for re-heating compressed air may be 
obtained and maintained; and this steam pressure need not be 
maintained with minute precision, as the temperature changes 
are not rapid. Thus with 100-lb. gage pressure the steam and 
water temperature would be a trifle under 340° while at 200 lb. 
the temperature would be 50° higher, and anywhere within this 
range the air would be heated to above 300°. 

A more constant steam- pressure and a steadier temperature 
may be maintained than here suggested, as is often done in house- 
heating arrangements, by means of automatic dampers and other 
fire-controUing devices, without blowing off or wasting the steam, 



188 COMPRESSED AIR PRACTICE 

or the hot water behind it, and all of the heat of the fire actually 
used would be that which went to the heating of the air. 

The better arrangement would be to have a large tubulated 
steam space and a small water space and to heat the air by the 
steam rather than by the water. The smaller body of water 
would enable it to be heated and steam generated quicker, with 
better command of th temperature fluctuations. 

It may be possiblr ^nat there are other combinations equally 
effective for re-heating compressed air, but that here suggested 
has the advantage of having been successfully employed in at 
least one notable instance. At the admirable compressed-air in- 
stallation at the mines of the Anaconda Copper Mining Company 
at Butte, Montana, where compressed air has supplanted steam 
for the driving of the big hoisting engines as spoken of in the 
preceding chapter, the air is re-heated by steam, and the addi- 
tional power gained by the re-heating is stated to be obtained at 
a cost of one-third of a pound of coal per horse-power hour. 



CHAPTER XVIII 
COMPRESSOR AND RECEIVER FIRES ND EXPLOSIONS 

It is a rather curious fact that the air-receiver, so far from 
being the air cooler which it is intended and assumed to be, 
has often been transformed into a combustion chamber and an 
air heater. 

A trouble all too frequent in compressed-air practice comes from 
the lubricants used in the air-cylinder, a combustible residuum 
from which accumulates in the air passages, upon the valves 
and in the air-receiver, often taking fire and causing sometimes 
disastrous explosions. 

Preceding or accompanying the deposition of this combustible 
residuum, the more readily volatilized constituents of the lubri- 
cants are liberated and mingle with or are carried along by the 
air. It is quite fortunate that there is a somewhat narrow limit 
to the relative proportions of air and of volatilized oil ingredients 
which will render the mixture explosive, a trifle too much or too 
little of the latter making the combination comparatively safe 
so far as instantaneous ignition and the sudden development 
of destructive force is concerned. When, however, the explosive 
proportions do occur, and the means of ignition also concur, 
then sudden and serious ruptures ensue with disastrous and 
often fatal results. 

While apparently the explosive mixture, even if combined in 
proportions within the explosive limits spoken of, is not likely to 
take fire of itself, but awaits the spark or flame to fire it, the solid 
or near-solid carbonaceous deposits do take fire spontaneously 
when sufficiently high pressures and temperatures are reached, 
the flames generally spreading rapidly in the receiver and in 
the pipes beyond, such flames sometimes traversing entire lines 
of piping, even reaching to the drills or other air actuated 
apparatus in mine or tunnel. 

This phenomenon was experienced several times in the work 
on the "New" Croton aqueduct about a quarter of a century 
ago. Quite recently a case was before the courts in the state 
14 189 



190 COMPRESSED AIR PRACTICE 

of Alabama where a man working about a pump driven by air 
was asphyxiated by the products of combustion delivered by 
the air pipe and lost his life. 

It has sometimes happened that such a flame as this has 
found somewhere in the pipes an explosive mixture of the right 
proportions and has fired it, with all the disastrous consequences 
of a real explosion. 

This matter may best be understood from the example of an 
actual occurrence in this line. A small air-receiver connected 
with a portable gasoline-engine- driven air compressor employed 
on street gas-main work exploded on June 24, 1912, near the 
entrance to Greenwood cemetery, Brooklyn, the occurrence 
exhibiting a number of interesting features illustrative of the 
conditions under which such ''accidents" may occur. 

To save time it may be premised that circumstances, after 
the event, indicated that in this case there was no sudden and 
enormous increase of pressure, such as would have been caused 
by the ignition of an explosive gaseous mixture, such for instance 
as occurs in the gasoline engine and furnishes its driving force. 
The normal working pressure usually carried in the receiver 
was probably not much if at all exceeded, but the receiver, 
having but a minute factor of safety at the best, was temporarily 
weakened under the abnormal conditions which arose, and the 
head let go. 

The receiver was suspended horizontally under the frame 
which carries the gasoline engine and the compressor. It 
was 6 ft. long and 2 ft. in diameter with dished heads, the convex 
side of the he^d being outward at one end and the concave side 
outward at the other end, this being the too familiar practice 
just for convenience in riveting in the last head, and for no 
other good reason. The air entered the receiver from the com- 
pressor about 2 ft. from one end of it and at the central height, 
and the air was taken out at the other side just opposite the 
inlet, see Fig. 54. A pop safety valve on the pipe leading from 
the compressor was set at 110 lb., and the usual working pressure 
was about 100 lb. 

As disclosed after the explosion, the entire interior surface 
of the receiver had been covered apparently with all the oil it 
could carry. If the compressor runner had been told to use 
all the oil he could, instead of as Httle as possible, the surfaces 
probably could not have been more oily. It would have been 



COMPRESSOR AND RECEIVER FIRES 191 

proper of course to have drawn off from time to time all the 
oil and water which might have collected at the bottom of the 
receiver, and this may or may not have been done, but it evi- 
dently would not have made the surfaces clean or have freed 
them from the gummed or thickened oil residuum with which 
they were coated. 

The condition of the inner surface (A, Fig. 54) of the head 
which blew out, the head with the convex face inward, was 
as different from this as could be imagined. It was dry and 
absolutely clean, and the color and condition of it showed that 
it had been red hot. It was evident that a local fire had raged, 
perhaps only for a minute or so, on this then oil- coated surface, 
burning the oil off and suddenly heating the plate, the fire not 
having had time to spread to the adjacent surface of the cylin- 
drical shell, and then the explosion came. 

In the act of explosion several curious things happened which 




Fig. 54. — Section of Exploded Air Receiver. 

it is worth while to note carefully. The head became so hot 
that, as it was under about all the air pressure it could stand 
anyway, the dishing of the head was suddenly reversed, and 
the originally convex surfa3e became concave, as Fig. 55 shows, 
this being a view of the inner surface. That this is the inner 
surface of the head is corroborated by the lip at the edge of the 
sheet. The ridge which is dented across the face was evidently 
caused by something against which the head struck, presumably 
the axle of the machine, as it flew off. 

We can readily imagine that this reversal of the dishing of 
the head took place with considerable of a snap, the shock and 
momentum, in addition to the aggregated air pressure, 45,000 
lb., being sufficient to cause the head to let go. 

When it did let go not a single rivet was sheared. It will be 
noticed in Fig. 55 that when the reversal of the dishing of the 
head occurred and the tearing-away-act was going on the 



192 COMPRESSED AIR PRACTICE 




Fig. 55. — Head that Blew Out with Bulge Reversed. 




Fig. 56.— End of Shell with Lip Bent Outward. 



COMPRESSOR AND RECEIVER FIRES 



193 



rivet-carrying lip of the head was bent outward all the way 
around, so that the opposite sides, instead of being parallel, 
stood off 30 to 45 degrees from the normal position, the end of 
the cylindrical shell to which they were attached bending out- 
ward in the same way. See Fig. 56. There were 80 rivets, 
3/8 in. diameter, and when the lip was bent out in this way these 
rivets just pulled their heads off, dropping them outside the shell 
all around, and then the rivets pulled out of the holes and the 
job was done. The fracture of the rivets was square across 
under the riveted heads and flush with the outside of the shell. 
Of the entire 80 rivets just 3 kept their heads on, and these took 
their leave of the shell by tearing notches out of it. 




Fig. 57. — Other End of Receiver. 



When the head flew off the body of the receiver naturally 
was driven in the opposite direction, but it could go only a few 
inches when it struck the axle of the machine, which dented the 
head as seen in Fig. 57. Besides this the shell seems to have 
received no other damage. 

The explosion seems to have been such as might have been 
expected from the normal air-pressure, and was not comparable 
in destructive effect with what it would have been if the receiver 
had been filled with an explosive mixture, such as we utilize, 
for instance, in the cylinder of the automobile. 

Undoubtedly receivers have often taken fire internally as 
indicated in this case, but having a, sufficient factor of safety not- 



194 COMPRESSED AIR PRACTICE 

withstanding the heating of the metal, they have been able to 
stand the strain and no accident has resulted; nevertheless the 
occurrence is one which it is not pleasant to think of, and every 
precaution should be taken to prevent it. It is not too much 
to say that every receiver should be made strong enough to stand, 
without bursting, the sudden heating of the head if the oily 
deposit within should take fire, because that occurs so frequently. 
If a receiver is said to be tested cold by hydrostatic pressure to 
165-lb. gage and then is guaranteed for a working pressure of 
110 lb., we have only a margin of safety of 0.5, this with every- 
thing cold and quiet. With a part of the receiver nearly or 
quite red hot and subject to the working shocks of the machine 
the onl}^ proper thing to expect should be the annihilation 
of the factor of safety and of the receiver with it. 

Only a few weeks after the occurrence here spoken of there 
appeared in Power an account of a fire of this character, but 
without explosion, in a vertical air-receiver which had the same 
kind of inwardly dished head at its lower end. After the fire 
got under way and the bottom of the receiver became visibly 
red hot the dishing of this head was reversed, so that the 
middle of the head bore on the ground, taking the entire weight 
of the receiver and lifting the projecting edges of it a couple of 
inches off the foundation. 

Chinese Air-receivers. — It might not be inappropriate to 
call the receivers of the type here spoken of ''Chinese" receivers. 
It is well known that the Chinese have never used barrels for 
the packing of their merchandise, and the reason that has been 
assigned for it is that they have never found a way of putting 
in the last head of the barrel without having a Chinaman inside 
to hold it up. 

Are we not in the same fix with our air-receivers, especially 
those of the smaller sizes which are the most numerous? These 
receivers are practicalh' all of the same pattern. They are 
made with the dished heads, which is eminently proper, and 
one head is placed with the convex face outward, which also is 
proper both for strength and looks, while the other head is set 
with the bulge of the head facing inward, a thing disgusting to any 
one with normal mechanical instincts, until he has become too 
familiar with it, and the only assignable reason for it is that they 
can't rivet the head in the other way without somebody inside 
to insert and hold the rivets. 



COMPRESSOR AND RECEIVER FIRES 195 

Rivets Should be Banished. — But why use rivets at all? May 
not the day soon come when we will realize the inefficiency of, 
and the absurd waste of material in, the riveted joint, and may 
not later the time come when riveting will be abandoned and 
samples of it become an exhibit in the museums? The writer 
lately stood with a young lady friend beside a high-pressure steam- 
boiler where the rivets were a protuberant feature. ''How 
strong that boiler must be with so many big rivets in it," she 
remarked. "Why, my dear girl, every rivet represents a hole 
in the sheets and that part of the boiler is the weakest instead of 
the strongest part of it.'' This, of course, is always true. With 
double the thickness of metal and the added weight of the rivet 
heads the joint is the weakest part and weakens the entire struc- 
ture by 25 or 30 per cent. And that is not all. The sheet is 25 
or 30 per cent, thicker over its entire surface, and the same per- 
centage heavier, than it need be to equal the strength of the joint. 

In our chapter on air transmission the welding of pipe lines 
is spoken of as having been successfully practised. It would 
seem that the practice should be equally applicable to air-receivers. 
The receivers are perhaps the most unsatisfactory detail of 
compressed-air practice to-day, and getting rid of the rivets and 
making receivers of uniform strength throughout should be a de- 
sirable achievement. The welded receiver should be stronger for 
the same weight, or lighter for the same strength, but more im- 
portant would be the possibility of placing both the heads cor- 
rectly, always assuming that a welding process is employed 
which can be operated entirely from the outside, as the pipe line 
spoken of was welded. 

How Receivers take Fire. — Every precaution should be taken 
to prevent the accumulation of oily deposit in the air-receiver, 
and not only should the receiver be drained at frequent and 
regular intervals, but it should also be examined and cleaned out 
at appointed times whenever the construction will permit. 
Supposing that there is an accumulation of water and oil in the 
receiver, the draining process may take out the water but leave 
the floating oil to finally cling to the surfaces which it may come 
in contact with as the water is leaving it. 

The interesting question remains as to the conditions causing 
or accompanying these ignitions of the oily surfaces, and there 
is usually more or less of mystery made of this phenomenon when 
it occurs. My thoughts carry me back to what was one of the 



196 COMPRESSED AIR PRACTICE 

familiar operations of the shop when I was every day in it, and 
that was the oil-tempering of steel springs. The spring is j&rst 
heated in the fire to a bright red and then is plunged into oil 
and cooled. This leaves it hard and brittle and it is drawn to 
the proper temper by ^'blazing off/' To do this the spring while 
dripping with oil is held over the fire, which must be without 
flame, and is slowly and carefully heated until the oil on the spring 
bursts into flame. The oil is not ignited by the flre. There is 
no flame or spark from the fire that does it; the flame simply 
comes of itself, as we might say, when the spring reaches the 
right temperature. When the spring reaches this temperature 
the oil will keep burning until the oil is burned off, or the spring 
while blazing may be dipped in the oil and the flame will go 
out, but if the spring is quickly drawn out of the oil so that it is 
not cooled much, the flame will '^ light itself" again as before, 
without being held to the fire at all. There is a certain tem- 
perature at which the oil will thus ignite spontaneously, and this 
temperature is so fixed, differing, however, for different oils, that 
it has been taken for generations as an index of the heat required 
for the tempering of springs, swords and other responsible articles 
of steel. 

Precisely the same thing that happens in the oil tempering 
of springs will happen to the oil-coated surfaces of air-receivers, 
pipes, etc., when they get hot enough. The oil will take fire 
of itself without the impulse of any spark or flame or other ex- 
traneous means of ignition. The oil-covered steel spring when 
the oil upon it takes fire in the open air is below a visible red heat, 
and that begins at about 700°. In the air-receiver whose explo- 
sion we have been considering the temperature due to compres- 
sion may have been as high as 500°. With air when compression 
begins at a temperature of 60°, the theoretical terminal tempera- 
ture after adiabatic compression to 105 lb. is 496°. This com- 
pressor was running in bright sunshine on a hot day arid the in- 
take air passed close to the already heated air-receiver before 
entering the cylinder, so that its temperature at' the beginning 
of compression was presumably above 100° and its terminal 
temperature above 500°. The fact that at this end of the re- 
ceiver there was nothing to cause any active movement of the 
air in contact with the oily surface may have offered special 
opportunities for the atoms of carbon and of oxygen to make 
love to each other and consummate their union. 



COMPRESSOR AND RECEIVER FIRES 



197 



We must remember that with the air at 105 lb. gage, it is 
compressed to 8 atmospheres, which means that the molecules 
— if that's the word — of carbon in the oil are individually ex- 
posed to or in contact with 8 times the quantity of oxygen that 
they would be in contact with if the air had only the normal 
pressure of 1 atmosphere, so that we may readily believe not only 
that the oil would be ready to spontaneously ignite at a lower 
temperature but that it would also burn more fiercely after the 
ignition. 




FiQ. 58. — Thermometer Record of Air Receiver Ignition. 



There recently appeared in Mines and Minerals, reprinted in 
Compressed Air Magazine, June, 1912, an account of an ignition 
of this character, but without damaging explosion, in connec- 
tion with a four-stage compressor employed for charging mine 
locomotives. The ignition occurred after the fourth stage of 
the compression, when the air was at a pressure of 65 atmos- 
pheres, and a recording thermometer which was providenially 
attached and in operation showed that the ignition took place 
at 275°, which seems to corroborate our suggestion that the 



198 COMPRESSED AIR PRACTICE 

higher the pressure the lower will be the temperature at which 
spontaneous ignition will occur. 

The thermometer record spoken of is reproduced in Fig. 58. 
The record with the compressor in operation begins at 10 a. m. 
The normal temperature when working before this had been 240°. 
At 11 A. M. the temperature began to rise a little and at 3 p. m. 
it was close to 280°. Then the oil stuff took fire and in a minute 
or two the temperature was above 600°. There was a fusible 
plug in the receiver or its connections which melted and blew 
out. The compressor was stopped, the fire went out at once 
for lack of fresh air and the temperature dropped back to where it 
was before. As there was immediate call for the air the com- 
pressor was started again at once and ran until 4 p. m. The 
remainder of the record is of no account as it was made after the 
stoppage. 

As to Quantity of Lubricant — No formal rules can be estab- 
lished as to the quantity of air cylinder lubricating oil that should 
be used in any given case, as the conditions may be so different, 
both as to the oil itself, the design and arrangement of the 
machine, the speed at w^hich it is run, the intake temperature of 
the air, the efficiency of the jacket cooling, etc., but one or two 
suggestive examples may be cited. 

A report was published in Power in 1911 of a month's running 
of the air compressors on the Panama Canal, and in that report 
it was shown that at Rio Grande a gallon of air cylinder lubri- 
cating oil was used for 3,360,124 cu. ft. of free air compressed; 
at Empire it was a gallon of oil for 4,317,716 cu. ft.; and at Las 
Cascadas for 5,163,936 cu. ft. 

It has only recently been reported that a compressor which 
has been running in the power house of the Nevada Consolidated 
Copper Company, 24 hours per day for five years past, is com- 
pressing 16,000,000 cu. ft. of free air per gallon of air cylinder 
lubricating oil, this rate of oil consumption being less than one- 
third of that at Las Cascadas, cited above. 

To get an idea of how this rate of oil consumption works out we 
may say that a compressor with an air cylinder 24 in. in diameter, 
which is quite a common size, and a piston speed of 400 ft. per 
min., will compress 1200 cu. ft. of free air per min., or 1,728,000 
cu. ft. per 24 hours. Then the oil used per day by this compres- 
sor, at the rate of 1 gallon per 16,000,000 cu. ft., would be only 
0.108 gallon, or less than a pint for the 24 hours. 



i 



COMPRESSOR AND RECEIVER FIRES 199 

In this run of 24 hours the cylinder surface swept by the 
piston would be 83 acres, and continuing the run until a pint of 
oil was consumed the cylinder area traversed would be 96 acres. 
It is safe to say that at this rate the accumulation of oil in the 
air receiver or pipes would not be very rapid. 

In sharp contrast with the above careful use of oil in air 
cylinders we have the following from a correspondent of the 
South African Mining Journal: 

''It has happened to me scores of times that I have had to 
leave the machines and come to the surface and ask the man in 
the power house to give the air receiver a tap, because the air 
has been so bad from the compressor oil, making everyone ill. 
I have known one mine where this used to occur regularly every 
week, the miner and all his boys suffering from bad headaches, 
caused by the gas in the air. I complained to the management 
on three occasions, and finally an investigation was made, and 
there were taken out of the air receiver 110 gallons of compressor 
oiir 



CHAPTER XIX 
SIDE LINES FOR THE AIR-COMPRESSOR 

There are ''side lines/^ or incidental employments for the 
air-compressor which are somewhat outside of its strictly legiti- 
mate line of work and which add considerably to the range and 
volume of its business. Air-compressors, without change of 
design or construction find frequent, extensive and constant 
employment in the compression of the various gases other than 
air. In fact compressed air and compressed gas distinctively 
so-called, whether it be natural gas compressed by the forces of 
nature, or manufactured illuminating gas artificially compressed, 
with other elementary gases which require manipulation or trans- 
fer in the chemical and other industries, are so similar in so many 
of their physical characteristics that it would seem to be more or 
less an ignoring of the functions and the adaptabilities of the air- 
compressor not to speak of its services, actual and potential in 
this field. For the air-compressor to be offering, and indeed 
urging the acceptance of its services for gas compression would 
seem to be natural enough, and it also would seem that it has 
some right to feel slighted that its proffered services are not 
more generally accepted, and that the greater gas interests give 
it so little to do when there is so much that it could do so well for 
the benefit of all. 

The Compressor and High-pressure Gas in Cities. — The sub- 
stance of what follows for a considerable portion of the chapter 
appeared some time ago as a personal contribution in a leading 
engineering journal of New York, and it is reproduced here with 
little change except a few additions. It needs not to be said that 
it deals with a matter which is not to be decided without thorough 
consideration and perhaps extensive discussion. It took shape 
in the mind of the writer without any recognized personal sug- 
gestion from any source, and it was submitted to the editor 
absolute^ without the knowledge of any tl ird person. So much 
for the responsibilit}^; if in any particular the writer errs; or if 
he is wrong all through, there are those who can and who should 
tell him so. 

200 



SIDE LINES FOR THE AIR-COMPRESSOR 201 

By what Right? — The title of the article was originally a 
^estion: ''By what Right?" and it was intended to fairly 
loharacterize what it introduced. The attitude of the writer 
upon this topic was, and is, interrogatory rather than assertive, 
although the latter may seem to be most suggested. It is ex- 
pected that it will appear to be high time for some one to be 
authoritatively propounding the question here crudely suggested. 

The simple question is as to the permissible retention of the 
ancient methods of gas storage and distribution, with special 
reference to the protuberant gas-holder, and this from the 
viewpoint of neither the gas producer nor the gas consumer, 
as such, but of the general and long-suffering public. 

Gas, of course, is an established necessity to practically all 
the people, and all questions as to cost of production and distri- 
bution, quality of gas furnished and convenience and relia- 
bility of service are to be settled between producer and consumer, 
with or without the aid of legal enactments, and no one else 
so far is interested. 

It happens, however, that the method of storing and distribu- 
ting the gas cannot be indifferent to the otherwise disinterested 
public, for it touches all at more than one sensitive point, and 
in an objectionable way which should not be tolerated or per- 
mitted, except in so far as it may be unavoidable. We have 
been so famihar for so many years with the sight of the supreme 
ughfier in every large outlook in every city in the land that we 
do not realize the unsightliness of it; we do not think to pro- 
test against it, or, in fact, in any way to question its presence. 
Who has thought of asking by what right the gas-holder intrudes, 
or has suggested its expulsion if its necessity and right are not 
proven and upheld? 

The question is so far from ever having been formulated that 
the gas-holder has never treated the public as in any was entitled 
to an explanation or a justification wherever and whenever it 
has chosen to plant itself. It has no doubt at times had to 
establish certain legal rights to locate, but always upon the unques- 
tioningly conceded assumption of the imperative necessity of it. 
Is it so necessary and indispensable? If so, it should be ''up 
to" the gas people to prove it in the light of the present century. 
When it came to proving the necessity of the telegraph poles 
they quickly fled the city streets. 

Few realize how bad the case is, or, indeed, have given the 



202 



COMPRESSED AIR PRACTICE 



matter any thought at all, and it would seem to be an opportune 
time to stir things up. Civic pride is becoming alert and 
restive. We are beginning to take an interest in the appear- 
ance and condition of our cities, and many movements are on 
foot for their betterment. But what shall we do with the 
gas-holder? Think of the costly viaduct starting from Grant's 
Tomb in New York and connecting to the upper stretch of 
the beautiful Riverside Drive all completed, and then almost 
immediately the popping up of the afreet we see in Fig. 59. 




Fig. 59.— By What Right? 

No one objected or thought of protesting at the time or since, 
as far as we have heard, because it is a gas-holder, you know. 

The work of redesigning, rearranging and permanently 
beautifying our cities, of rendering them more satisfactorily 
habitable, so that not only we, the indwellers, but also the in- 
comer and the transient onlooker, shall say it is good to be here, 
cannot proceed far before we reahze that much of our doing 
must first of all be undoing. We cannot re-arrange and upbuild 
to our liking until we tear down, banish or obliterate the things 
which, if undisturbed, would render our efforts futile. Objec- 
tionable and, but for our familiarity with them, often disgusting 



SIDE LINES FOR THE AIR-COMPRESSOR 203 

features have accumulated and established themselves un3hal- 
lenged, and yet if they are allowed to remain there can be no 
real progress toward permanent and satisfactory improvement. 




Fig. 60. — ^Looking up West End Avenue. 

In Fig. 60 we are looking up West End Avenue, New York, a 
beautiful and high-class residence street which retains its select 
character all the way up to the end of it, two or three miles to the 



r^^^^l^^ 



lllfirftfiiiirari 



mM- 



Fig. 61. — Looking up West End Avenue. 

north. In Fig. 61 we are still looking up West End , Avenue but 
from a point just a quarter of a mile further down. From the 
same point, turning to the right, we have Fig. 62, a row of well- 



204 



COMPRESSED AIR PRACTICE 



built tenements, but only colored people can be found to occupy 
them. Fig. 63 was their outlook when this was written. Since 
then these unimproved lots, there being apparently no prospect 




Fig. 62. — Tenements facing the Gas Holders. 

of erecting respectable, substantial, permanent buildings upon 
them, have mostly been covered with cheap and shabby sheds 




Fig. 83.— The Outlook from the Tenements. 

for storing carts, etc., which city ordinances do not permit to 
stand in the street at night. Directly opposite these lots, on 
the other side of West End Avenue, are the primitive rocks of 



SIDE LINES FOR THE AIR-COMPRESSOR 205 

Manhattan, with squatter shanties surmounting them, neither 
of which, shanties or rocks, it has been worth while to remove. 
The next block to the north on the same side of the avenue is 
a lot, without buildings, in which castings and steel work are 
stored. 

Farther away in all directions, for, say, three or four blocks 
all around these gas-holders, they have been the means of accom- 
plishing, as some might say, a work of great beneficence by so 
depreciating the property values as to make posejble the erection 
all through the neighborhood of tenements of the cheapest class 
for the occupation of the minimum wage-earners and of the strug- 
glers for precarious subsistence. If it were not for the blessed 
gas-holders where would these poor people go? 

This is not in the outskirts of the city, but in the heart of it, 
the location being in the Sixties, while the island is solidly built 
up for more than a hundred blocks above. There is everything to 
warrant the presumption that all this section of the city, of which 
this one group of gas-holders is the center, would be very differ- 
ently occupied and improved if the gas-holders were not there. 
Certainly it would all be well and profitably used, which it is not 
now, and the location, otherwise desirable and easily accessible, 
deserves a better fate. 

We could get pictures of similar character to those here pre- 
sented from each of the dozen or so of gas-holder neighborhoods 
of Manhattan, and the same of every other large city, showing 
them all to be nuclei of desolation and responsible for the depre- 
ciation of property values amounting in the aggregate to hundreds 
of millions of dollars. It is not for the present writer to estimate 
the amount of this depreciation, but it would be well for real 
estate experts to be doing some figuring upon the problem. 

Suppose that some day there should come to some one the 
assurance in advance that the gas-holders in the cities would all 
have to go (and the gas companies are likely to be themselves the 
first to realize it), what an attractive and promising speculation 
it would be to quietly buy up all the depreciated property in 
these gas-blighted neighborhoods. 

The gas-holder is simply to-day the survival of the unfit, if 
not of the unfittest, and it seems more tenacious of life than any 
other thing of which we have record. Nothing can be more 
certain than that if the gas business were beginning as a new busi- 
ness to-day it would not begin with the absurdly low-pressure service 

15 j 



206 COMPRESSED AIR PRACTICE 

now in use and it would not use the big gas holders; but it began in 
that way a hundred years ago and has not changed. 

"Little of all we value here 

Wakes on the morn of its hundredth year 

Without both feeling and looking queer," 

and low-pressure gas is queer enough. 

Just think of it. Ordinary city gas is transmitted and stored 
and distributed at pressures so minute as not to be measurable 
in pounds to the square inch, as we commonly measure and record 
pressure, or even in ounces, but in tenths of an inch of water. 

We happen to have conveniently at hand the figures from a 
tA-pical city plant. At S>Tacuse, N. Y., they have about 170 
miles of gas mains, from 2 in. to 20 in. in diameter, and the gas 
pressure varies from 112 in. to 3 1 2 in. of water, these being 
the limits, or, say, 1 20 to 1 8 lb. to the square inch. In the 
literature of the gas men the maximum pressure here mentioned 
is spoken of in the record as 37 tenths of an inch of water. Why, 
a boy with a tin bean blower could give you double that pressure. 
A familiar boj-'s trick is to blow into a burner against the city 
gas-pressure, filling the pipes with air and putting out the lights. 

And yet, the existence of the gas-holder is absolutely con- 
ditional upon the retention of these low pressures, these pressures 
of a hundred years ago. in storage and in distribution. Any, 
even a slight, increase of pressure would be death to the gas-holder 
at once. Take any one of the largest gas-holders, such, for in- 
stance, as the first one shown here, and an increase of 1 lb. in the 
pressure within it would require an addition to the weight on the 
top of about 2000 tons. This would give a steel top over 3 in. 
thick, an effective armor against aeroplane bombs. 

It would, in fact, be impossible, for another reason, to carry 
an additional pound of pressiue in the gas-holder, even if it could 
be weighted down sufficiently. In all candor and seriousness, 
the modern gas-golder is a magnificent achievement in engineer- 
ing, and one of the wonders of it is the telescoping feature. When 
the holder is fuU and has risen to the top of its guides it is not, 
as it looks, a single shell, but consists of four or five ''lifts," which 
shde into each other as the}' descend. To make a gas-tight 
joint between the lifts there is to each a ''water seal" which 
retains the gas with absolute security', as long as it holds it at all, 



SIDE LINES FOR THE AIR-COMPRESSOR 207 

but if the gas pressure were increased to 1/2 lb. to the square inch 
or about that, instead of 1/8 lb., the present maximum, the water 
would all be blown out of the ''seals" and the gas would escape 
as fast as it flowed in. 

It is, of course, familiar to everyone that the rate of gas con- 
sumption varies throughout the entire 24 hours, what is called 
the ''peak" load coming between sundown and midnight, with 
a smaller peak in the morning. When the peak is on, the con- 
sumption is, of course, several times as great as, for instance, in 
the small hours when the day is young, and a pipe transmission 
which would be sufficient if it could be continued uniformly all 
day and all night is altogether unable to maintain the supply 
when the demand is greatest. 

It is said, therefore, and this is the special excuse for the added 
monstrosites of recent years, that we must have the big gas- 
holders to take care of the peak load.' Certainly, if we retain both 
the low-pressure transmission and the low-pressure distribution. 
With the 2 or 3 in. of water pressure the gas cannot be rushed 
through the pipes. With a pressure increased to only 15 lb. 
to the square inch the volume of the gas would be reduced one- 
half, and it could be driven along at more than four times the 
present speed, so that pipes of the same size as now in use would 
transmit eight times the quantity of gas, or as much in three 
hours as can now be sent through in the 24 hours. This surely 
would be at a speed sufficient to take care of the peak load, and 
supply all consumers at all times without the waiting in gas- 
holders by the way. In this way we have at once a suggestion 
for the beginning of reform by the warrant it gives for first of 
all insisting that no additional gas-holders shall be erected any- 
where for taking care of peak loads. We have already a long 
list of locations where gas is transmitted at high pressures to 
reinforce existing low-pressure storage systems and avoid the 
necessity of increased holder capacity. 

A Mr. Jones, before the Pacific Gas Association, is thus re- 
ported: "One of the ambitions of my life is about to be realized 
in the construction of a steel bracelet around the city of San 
Francisco for feeding the low-pressure system. This main is 
now in the ground and is 16 in. in diameter and 7 1/2 miles long. 
It extends from the old Portrero Gas Works around the city to 
the old plant we call the North Beach Station. The line is not 
yet in use for conveying gas, on account of construction work 



208 COMPRESSED AIR PRACTICE 

now going on, but it has been under 60 lb. pressure for over 30 
days, and has maintained a constant pressure at uniform tempera- 
tures both day and night." The piping was entirely successful 
for the purpose intended, and the preliminary test gave full 
assurance that there would be no leakage. 

What we are certainly coming to is the entire abolition of 
the hundred-year-old gas pressures, with the gas-holders which 
cannot survive them, and the service of gas at so-called high 
pressures — although they would not be high as compared with 
steam- and compressed-air pressures — directly to every consumer. 
The following from the Gas World (Feb. 4, 1911), an English 
publication, is reprinted with approval by the Progressive Age 
(March 1, 1911), an able representative of American gas interests. 
As will be noticed, it goes far beyond the suggestions of the pres- 
ent writer. The article referred to says : 

''The introduction of high-pressure gas, when thoroughly 
understood, will do more for the industry than ever the incan- 
descent mantle did. Its potentialities — its far-reaching utilities 
— are beyond all power of description. 

"All great changes take place gradually, and it is not to be 
expected that the change from low- to high-pressure gas will be 
any exception to the rule. Engineers will not jump from 2 in. 
of water to 200 lb. to the square inch, and yet this is the jump 
which modern improvements enable any man to take who 
seriously looks into the question and who realizes what is at his 
disposal to carry it out. 
''With regard to experience, we have at our disposal the record 
of railway carriage lighting by compressed gas up to 150 lb. or 
more. In America gas has been distributed at 200 lb. In this 
country (England) gas has already been distributed to 100 lb., 
and several miles of mains will be in actual use before many 
weeks." 

The article quoted then goes on to consider the different dis- 
tribution of costs under the new system, which we need not go 
into here. Although the high pressures it refers to are all matters 
of actual record, and in natural gas transmission the pressures go 
much higher, it would be sufficient :^or our present purpose to 
have only 15 lb. per square inch as a maximum working pressure. 
This would surely render the gas-holders worthless, and if suffi- 
cient pressure were put upon the outside of them by the awakened 
public they would collapse and disappear, property values would 



SIDE LINES FOR THE AIR-COMPRESSOR 209 

reassert themselves over the desolated city areas, and there would 
be renewed hope for other reforms to follow. 

The gas-holder, it may be suggested, is, in a way, like our bad 
spelling, as some call it; our bizarre weights and measures, as the 
metricists insist; our Fahrenheit thermometer; our decimal, 
instead of duodecimal, notation; a thing which started wrong, 
but which has now become so established that change is not to be 
thought of. In this case a change insists upon being thought of. 

It is not necessary to remind anyone that no gas-holders of the 
gravity- pressure type are used, or could be used, in the distribu- 
tion of natural gas, so that they cannot be imperative for artificial 
gas. As we have seen, they at once become impossible with any 
increase of pressure; yet gas consumers are requiring higher 
pressures. The obsolescent fish-tail burner was satisfied with a 
pressure of 1 1/2 in. of water; the incandescent mantle gives much 
more light for gas consumed, but it demands higher pressures. 
Higher pressures are called for where gas is used for heating pur- 
poses and much higher pressures are required for gas engines. 
Gas should be brought to each consumer at a pressure high 
enough to require a regulator, and this could be individually 
adjusted to any pressure required, so that everyone could be 
using it at its best, according to the use to which it was applied. 

Object-lesson in High-pressure Gas Distribution. — Instances 
are now becoming numerous of the distribution of gas, and of 
its delivery to consumers at pressures above those which are 
possible with the famiUar district gas-holder, so that it is necessar- 
ily discarded. Here, for instance, is some account of a recent 
installation of the St. Louis County Gas Company, St. Louis, 
Missouri. 

There are gas holders of the familiar type at the gas generating 
plant, where the pressure maintained is from 5 in. to 9 in. of 
water, the latter being the maximum pressure reached when the 
tank is full. These tanks are a convenience and a factor of 
economy, possibly they are a necessity, and in either case if 
properly located there can be no objection to them, but no other 
gas-holders are employed for the entire distribution system. 

The compressors take the gas at this gas-works pressure and 
compress it to a maximum of 40 lb. gage. The pipes into which 
the compressed gas is delivered have a capacity of 20,000 cu. ft., 
and these constitute the entire storage for the gas after leaving 
the compressor. As a pressure of 10 lb. is sufficient for all pur- 



210 COMPRESSED AIR PRACTICE 

poses, the permissible range of pressures in this pipe system, 
about 2 atmospheres, allows fluctuations in the quantity of 
gas contained of about 40,000 cu. ft. of low-pressure gas, although 
neither limit of pressure is actually reached in practice. The 
gas comes to the consumer at whatever may be the pressure in 
the pipes at the time, and then it passes through an individual 
pressure reducer, after which it is metered at the constant low 
pressure maintained. 

This is not to be considered as in any respect an experiment, 
at least with this company, as they have been following this sys- 
tem of distribution for some years, and find that it not only gives 
satisfaction to all, but that it pays well. They have never ex- 
perienced any trouble from deposits of any kind in the pipes and 
the gas is found to be practically as rich after compression and 
transmission as before. There also has been found no trouble or 
danger from the heating of the gas in the compressing operation, 
and there has been no accident of any kind. 

In Fig. 64 we see the interior of the compressor room. In one 
corner of the room, but not seen in this view, there is what is 
apparently a vertical receiver such as the familiar accompani- 
ment of the air compressor, but in this case it is, instead, a tar 
extractor through which the gas passes before entering the com- 
pressors. These machines are two duplex gas-compressors with 
cross-compound steam-cylinders 12 in. and 23 in. in diameter, and 
duplex tandem gas-cylinders 17 1/4 in. in diameter with a com- 
mon stroke of 18 in. and a normal speed of 120 r.p.m. The gas- 
cylinders are, of course, completely water-jacketed. The mean 
horse-power is about 120 and the gas-compressing capacity, with 
liberal allowances, about 1,500,000 cu. ft. per 24 hours. This 
approximates the present producing capacity, but considerably 
exceeds the consumption. 

The compression of the gas under the conditions here presented 
is a very simple, or as we might say, a very comfortable job for 
the compressors. The piston inlet furnishes an ideal means for 
connecting the intake, and the only automatic control required 
is a speed regulator, as on a stationary engine. This can easily 
be adjusted for different speeds according to the rate of gas 
consumption. 

The gas is delivered into an 8-in. main, from which there are 
branches or continuations of 6-in., 4-in., 2-in., and 1 1/2-in. 
pipes, the aggregate length of which may be inferred from the 



1 







s 

o 
O 



w 



SIDE LINES FOR THE AIR-COMPRESSOR 211 

fact that the area served is about 120 square miles. The present 
number of customers is 6000, which number is being increased 
as fast as the pipes can be laid, the present daily output of gas 
being approximately 500,000 cu. ft. Only one of the two 
compressors is as yet required, and this is run 16 hours a day. 

Fig. 65 is an accurate reproduction of an actual official 24-hour 
record disk from the recording pressure gage located near the 
compressors at the beginning of the 8-in. pipe line. This record 
reads from 8 a. m., Nov. 25, 1911, to 8 a. m., Nov. 26. The 




Fig. 65. — High-pressure Gas Record. 



pressure line on the record card coincides almost exactly with 
that of another gage 2 1/4 miles away at the other end of the 
8-in. line, so that it is not necessary to reproduce the latter. The 
combined testimony of the two disks assures us how little loss of 
pressure there is in this transmission of over two miles, and sug- 
gests that the same pipe may easily transmit twice or three times 
the present volume of gas. 

The record line of the pressure-gage card tells its story very 
clearly. The record begins a little after 8 a. m., and at 9 a.m. 



212 COMPRESSED AIR PRACTICE 

everything is running smoothly, the output of the compressor 
evidently keeping pace very closely with the eight mid-day hours 
to 5 p. M., a slight loss of pressure appearing about noon, when we 
may assume that some extra gas is used for cooking purposes. 
At 5 p. M. the demand for both lighting and cooking causes the 
pressure to fall quite rapidly for the next two hours. This is 
when the ''peak" load occurs, the peak being represented on the 
diagram by a depression. 

Before 7:30 p. m. the compressor output has caught up with 
the consumption. Then the pressure rises gradually until 9:15 
and is nearly stationary until 10:15, when there is a rapid rise 
until 11 : 30. The upper working limit is nearly reached here and 
the compressor is stopped, as indicated by the sharp angle at 
11:35. From this point until 6:50 a. m. the compressed gas in 
the pipes is sufficient to supply the demand. The drop is very 
rapid after 6 a. m. but after the compressor is started the line 
rises easily before 9 a. m. to the normal day working pressure 
of about 30 lb. The pressure where the record ends at 8 a. m. 
seems to be about 2 lb. higher than on the preceding day; perfect 
coincidence, of course, was not to be expected. 

The system of high-pressure gas distribution is constantly 
extending, both in this country and in Europe, and entirely upon 
a business basis. It is found that the cost for installation, oper- 
ation and maintenance of compressor and high-pressure piping 
is less than that of the much larger low-pressure pipes, the dis- 
trict gas-holders and the land actually required, to say nothing 
of the depreciated land values which the community has to stand 
in the vicinity of gas-holders. 

The plant here spoken of is a comparatively small one. That 
the principle it embodies is practically applicable to much larger 
service is self-evident. That it is not applicable and that it will 
not eventually be applied to all gas service, however vast or 
concentrated; it is not safe to assert. 

Compressing Natural Gas. — It is not easy to realize the enor- 
mous aggregate capacity of compressors employed in natural gas 
transmission service. Not only are the total volumes very large 
but the pressm-es dealt, with also are much above those with 
which we are familiar in ordinary compressed air practice. Nat- 
ural gas is now produced in many localities and the transmission 
extends to great distances in many if not most of the gas districts. 

The compressor would seem to have a claim upon every gas 



SIDE LINES FOR THE AIR-COMPRESSOR 213 

well for ultimate employment. The pressures are generally high 
to begin with, and then the gas can laugh at the compressor if it 
has not too far to go; but as the pressure invariably falls off there 
comes a time when the compressor must be called on to boost. 
Then the distances to which the gas must be transmitted are 
often so great that no matter what may be the initial pressure 
the help of the compressor is needed before the gas can be relied 
upon to reach the most distant consumer. 

A typical case of commercial gas transmission of considerable 
proportions is that of the City of Cincinnati, Ohio, and the sur- 
rounding towns, the brief sketch of which here given being made 
up from information furnished by Wm. A. Miller, Gen Mgr. Gas 
Dept., Union Gas and Electric Co., Cincinnati. 

The pipe line from Culloden, W. Va., to Cincinnati, Ohio, 
consists, tracing it backward, of 123 miles of 20-in. seamless 
steel pipe, from Cincinnati to the Big Sandy River compressing 
station, and 33 miles of 18-in pipe from the Big Sandy to the end — 
or beginning — of the main line near Culloden. From the 18-in. 
main pipe line there are about 40 miles of lateral 12- and 8-in. 
pipe lines extending to and penetrating the gas fields which em- 
brace about 300,000 acres, on which are drilled about 100 gas wells 
having an open flow of approximately 200,000,000 cu. ft per day. 

The West Virginia pipe line, used in connection with storage 
holders (capacity, 10,000,000 cu. ft.) is capable of transmitting 
to the consumers upward of 75,000,000 cu. ft. of gas daily, with 
an initial pressure of 320 lb. at compressor station, and a term- 
inal pressure of about 60 lb. at Cincinnati. 

The distribution system of the City of Cincinnati and villages 
adjoining, consists of about 45 miles of belt line in the form of a 
figure 8, outlining the whole territory to be supplied, upon which 
a pressure of from 3 to 5 lb. is maintained, varying with the 
demand for the gas and the temperature of the atmosphere. 

From the belt line the gas is caused to pass through forty-two 
district regulators set in masonry pits, and located in streets in 
different parts of the city and villages supplied, which regulators 
are adjusted for maintaining a pressure of 5 oz. on their outlets. 
The outlets of the regulators are connected to the low-pressure 
system of mains, consisting of about 650 miles of all sizes of main 
pipes, from 4 to 30 in. in diameter. Individual house regulators 
are only used where the belt line is the only main from which a 
supply of gas can be obtained for the customer to be supplied. 



214 COMPRESSED AIR PRACTICE 

Information somewhat more detailed as to natural gas com- 
pressor practice is here abstracted from a paper in the proceedings 
of the Engineers' Society of Western Pennsylvania by Mr. E. D. 
Leland. 

It is stated that in 1892, at Greentown, Indiana, there was 
completed the first station designed for compressing large 
quantities of natural gas to extremely high pressures. The 
problem was to continuously deliver at Chicago an adequate 
supply through two 8-in. lines 120 miles long. The compressors 
being installed a considerable time before the pipe lines were 
ready, they were first used to compress the gas into large steel 
tanks under a pressure of 700 lb., and these tanks were shipped to 
Chicago. 

The pipe lines when completed were tested at 600 lb. air-pres- 
sure, and the entire station was designed for these high pressures. 
The ordinary delivery pressure was somewhat lower, but the com- 
pressors proved amply able to compress the gas up to the maxi- 
mum pressure whenever required. 

As storage capacity for a sufficient reserve in or near Chicago 
was entirely out of the question, it was most essential that a 
continuous delivery should be maintained by the pipe lines and 
the compressing station alone. Hence the installation consisted 
of twelve straight-line, single-stage, steam-driven compressor 
units of moderate size, so that an accident to any one machine 
would not seriously affect the delivery capacity of the station. 
Later in the history of gas compressing larger and fewer units 
were used. As early as 1896, the Fort Wayne Gas Company 
installed some large cross-compound Corliss engines which were 
used until the practical exhaustion of the Indiana gas field, and 
these were then sold for use in the Ohio and Kansas gas fields 
where they give good service. 

Single-stage compression was satisfactory for the twelve 
machines spoken of above as long as the high intake pressure was 
maintained; but when the pressure fell off two-stage compression 
was resorted to, using eight of the machines for the first stage 
and the other four for the high pressure and delivery work. 

A gas compressor is practically an air- compressor designed to 
operate under the high pressures usually required in natural gas 
transportation, and no experimenting is now required in order 
to obtain reliable compressors, nor can there be any valid excuse 
for failure to deliver gas on account of defective machinery. An 



SIDE LINES FOR THE AIR-COMPRESSOR 215 

instance is cited of four compressors installed in 1904 which have 
been in almost constant operation ever since. They have been 
steadily compressing gas to pressures ranging from 200 to 275 lb., 
and are still operating successfully with the original intake and 
delivery valves that came with the machines. 

In some cases the high-speed gas engine, connected by rope 
drive to a slower speed and longer stroke compressor, proves a 
good arrangement. It not only affords relief from the undesir- 
able features of the short-stroke compressor, but it also makes 
it feasible to locate the engine at a safe distance from the compres- 
sor. While the necessarily quick starting of a gas engine is more 
liable to cause trouble, by breaking the ropes or by throwing 
them out of their grooves, than is the case with the slower starting 
steam engine, still with this type of plant fairly good results have 
been obtained. 

For continuous and reliable service in compressing large quan- 
tities of gas, the modern cross-compound Corliss engine, directly 
connected to the compressing cylinders, makes an ideal installa- 
tion. This engine is particularly well adapted for the long stroke 
and moderate speed, so engineers thoroughly understand that the 
changing pressure conditions in field and main lines are also well 
met by the flexibility and high overload capacity of this type of 
prime mover. So well is this fact recognized that in the stations 
delivering gas to the Pittsburgh district we find 39 Corliss engine 
driven compressors installed, comprising a total maximum capac- 
ity of over 76,000 h.p. 



CHAPTER XX 
GASOLINE BY COMPRESSION— LIQUEFIED NATURAL GAS 

The present chapter has to do with an industry which has been 
developing rapidly, and the details of which have been becoming 
matters of general knowledge, while the preparation of this book 
has been in progress, and it cannot expect to be entirely up to 
date while developments and revelations are still occurring. 
The demands of the automobile and the auto-truck have advanced 
the value of gasohne enormously, and have stimulated the search 
for new sources of supply. Under this impulse there has been 
growing a new industry which is understood to be proving highly 
remunerative and to have the merit of conserving one item of 
our natural resources by producing value and usefulness from 
one of the waste and hitherto unconsidered effluents of the oil 
industry. 

The general public has understood that there are oil wells and 
gas wells, but in fact many wells are, and most wells have been 
or may become at some time, either or both of these. So also the 
public has known of in a general way, and has deprecated, the 
enormous wastes of both oil and gas from their respective wells 
through the failure to provide the means of conveying or of 
storing, and thus of ultimately utilizing all of either product 
which has flowed forth too copiously when the wells have been 
first opened, or of choking off the flow until arrangements could 
be made for its economical disposal. Both individual or corpor- 
ate enterprise and legal enactments have made such waste less 
frequent now. It often happens, however, that when the gas 
comes as the accompanying product of a producing oil well, or 
when the gas continues to flow after the pumping of oil has ceased 
such gas all goes to waste because it is thought that it will "cost 
more than it comes to" to save it. It appears now that there is 
money in it, and also that money is being got out of it. The 
following account is mostly obtained, condensed and rearranged, 
from a full and clearly written description in Mines and Minerals. 

Most oil wells at the beginning are self-flowing, or are even 

216 



GASOLINE BY COMPRESSION 217 

gushers, yielding five or six hundred barrels a day, and in Cali- 
fornia much more than that, but the decrease in production 
is rapid, they become pumpers and then are pumped only at 
intervals until they finally yield less than a barrel a day. The 
opening of new wells goes on continually and the number of 
abandoned wells is very great and always increasing. Al 1 the 
time, and long after the yield of oil has ceased there is a flow of 
gas from the wells, and it is this gas which in later years has made 
most of the waste. 

The beginning of the recovered gas industry began really with 
the gas piped and thus saved itrom oil wells rather than from the 
entirely abandoned gas. In the long pipe lines it has been a com- 
mon occurrence for gasoline to collect wherever a down bend 
in the pipe has made a pocket. Water also is deposited in the 
pipes and the re-evaporation of the gasoline through leakage or 
otherwise may so reduce temperatures as to cause annoyance and 
serious trouble by freezing the water and choking the pipes, and 
in seeking a remedy the process for manufacturing gasoline from 
the gas has developed and since its perfecting on practical lines 
it is being applied to the abandoned gas-yielding oil wells above 
spoken of. The plants now employed, numerous and not in- 
dividually very large, are equipped with a refinement of appartus 
and method which has been perfected after considerable experi- 
menting and selection. 

The cut, Fig. 66, shows the layout of a typical West Virginia 
gasohne plant with a capacity for treating 150,000 cu. ft. of gas 
in 24 hours and producing 500 to 800 gal. of gasoline at 92° 
Baum6. The gasoline is shipped in 50 gal. steel barrels which are 
hauled by wagon to the nearest railroad station, and the gas also 
no longer flows to waste but is piped to the nearest natural gas 
pipe line, usually not distant. 

The gas is compressed in two stages, or successively by two 
35-h.p. gas engine driven, straight-line air-compressors. The 
first compressor which may have a cylinder from 6 to 12 in. 
in diameter draws the gas from the piping system connecting all 
the available wells in its immediate neighborhood, and from an 
intake down to, say, 15 in. of vacuum the gas is compressed to 
20 or 30 lb. It then passes through a water cooler to the second 
compressor with a cylinder diameter of, say, 4 1/2 in., where it 
is compressed to 150 lb. or over. This finaKpressure must bo 
determined by trial, as the process depends considerably upon 



^PRESSl 



mA CTICE 




GASOLINE BY COMPRESSION 219 

the quality of the gas and the refrigerating treatment. In some 
plants more gasoHne is produced with a terminal pressure of 
150 lb. than with 250 lb. 

The gas at 150-lb. pressure passes through an 80-ft. water 
cooler, and then through a double 80-ft. gas cooler, the latter 
using the by-product gas for cooling. The saturated refrigerated 
gas under a pressure of 150 lb. or over, enters the side of the accu- 
mulator tank at a point about two-thirds its height, measured from 
the bottom. A baffle plate riveted in the tank deflects the flow 
and precipitates the gasoline. The accumulation of gasoUne is 
shown by a gage glass, and periodically the attendant blows it 
into a storage tank of 120-barrels capacity. The storage supply 
stands at about 20 lb pressure. From the stock tanks the gaso- 
line is loaded into the steel barrels. 

The crank case on some of these engines is closed and has a 
vent pipe leading above the building; thus, any gas leaking from 
the cylinders will be carried out of the building and the danger 
from fire or explosions is lessened. The make-and-break spark 
system is used for ignition, and a friction-driven magneto for 
each engine is located in a small building some 100 ft. distant. 
A small gas engine operates these magnetos and also the generator 
for lighting the plant. An air starting outfit, consisting of one 
air pump compressing to 150 lb., air-receiver, starting valves, 
pressure gages, etc., makes the starting of these large gas engines 
an easy and a safe operation. About 2 lb. gas pressure is used 
for the gas-engine service. A regulator placed outside the build- 
ing is necessary to deliver the fuel at a constant pressure. 

The Cooling System. — All the gas-engine and compressor 
cylinders are water-cooled. The gas as it comes from the wells 
may be at about 60°. The heated gas from the first compression 
goes to a water cooler consisting of a concrete vat 20X4X4 ft. 
A continuous flow of cool water passes through this tank in which 
a 10-in. pipe is laid lengthwise along the middle and to which the 
3-in. delivery pipe from the first compressor is attached at one 
end, while to the other end an inlet pipe to the second compressor 
is attached. 

The high-pressure gas from compressor No. 2 passes first 
through an automatic separator which removes any lubricant 
that may be carried over from the compressor. It also catches 
any gasoline that may drain back from the second cooler. This 
second cooler is 80 ft. long and consists of a concrete tank like the 

16 



220 COMPRESSED AIR PRACTICE 

intermediate cooler. A 10-in. pipe is placed lengthwise of the 
tank. The 2-in. delivery pipe enters a special fitting at the rear 
end of the 10-in. pipe, and the water-cooled gas passes out by 
a 3-in. pipe 80 ft. distant. This 3-in pipe enters the end of an 
80-ft. length of 6-in. pipe, and returns through a second 80-ft. length 
of 6-in. pipe and thence to the accumulator tank. From the 
gas space in the top of the accumulator tank a pipe leads to a 
gasoline trap which collects any gasoline that may be carried over 
from the accumulator and returns it to the stock tank. This trap 
is also fitted with a pop safety valve that relieves the accumulator 
from any over pressure and delivers the gas that may be blown 
off to the fuel-supply gas mains. Above the gasoline trap the 
by-product gas passes through a reducing valve and enters at 
low pressure the lower 80-ft. branch of the 6-in. gas cooler. This 
No. 3 cooler is made up of a loop of 6-in. pipe laid in a box packed 
with sawdust. 

The peculiar design of the cooling system makes it necessary 
to use some specially designed pipe fittings. The cooling effect 
of the expanded by-product gas is considerable, as it flows 160 ft. 
through the 6-in. pipe that encloses the 160 ft. of 3-in. pipe carry- 
ing the compressed gas to the accumulator. From the 6-in gas 
cooler, the expanded gas goes to the 80-ft. water cooler and passes 
through a 3-in. pipe laid lengthwise through the center of the 10- 
in. pipe. From cooler No. 2 the by-product gas goes through a 
2-in. pipe to the gas engine feed-line. The by-product gas not 
used by the plant goes into the natural gas mains and is sold. 
The overflow water from the concrete tanks flows by gravity to 
the water-jackets of the engines and compressors. The by- 
product gas is a blue-flame gas that is more desirable for fuel and 
lighting than the raw gas, as it does not deposit any soot or 
blacken at all the furnaces, gas mantels, cooking utensils, etc. 

The partial vacuum produced by the first compressor as it 
draws its gas supply from the wells aids both the oil and the gas 
production; in fact, in some cases the gas is given to the gasoline 
plants, by the oil man, as the increase of oil due to the vacuum is 
quite an item to the well owner. At the same time, however, 
the owner draws back all the by-product gas he needs for pumping 
the oil. 

The vacuum in the field lines is one of first importance in gaso- 
line production. So great is this feature that in some gasoline 
plants a special independent low-stage vacuum pump and gas 



GASOLINE BY COMPRESSION 221 

compressor has been installed so as to regulate the field pressure 
and increase the production. With this low-pressure compressor 
24 in. or more of vacuum can be held on the wells, and at the same 
time the efficiency of the regular compressor units not be lowered. 
The advantages observed from the use of the vacuum system 
have led to the reopening of abandoned oil fields, not for the oil 
but for the gas from which they make gasoline. 

The business of making gasoline from natural gas is necessarily 
a hazardous one. It is a new business, and this, coupled with the 
usual combinations of ignorance and carelessness, makes a list 
of accidents that one would naturally expect. Even the empty 
barrels have exploded when standing exposed to the hot sun and 
with the vent plugs set tight. On one occasion an empty went 
up when standing on the freight station platform. Those old in 
oil-well service have become accustomed to handling nitroglycer- 
ine, and while they respect it, they treat it with a feeling of 
contempt. On the other hand, gasoline of from 92° to 100° 
Baum6 is a new thing to them, and they have got *' burned'' as 
a consequence. Therefore, to hear an operator about a gasoline 
plant remark that he would rather carry ''nitro" than gasoline 
is evidence of the fear in which it is held. ' 

Portable Liquid Gas. — The preceding may be said to merely 
outline what may be considered the crude beginnings of an 
industry which under cooperative scientific investigation and 
ingenious application of the principles discovered has resulted in 
a successful method of Hquefying natural gas and transporting it 
for use, especially in isolated localities. Dr. Walter O. Snelling, 
who has been a leader both in the experiments and investigations 
preceding and in the applications resulting, gives a summary of 
the results of the new processes in a paper before the Pittsburgh 
Section of the American Chemical Society. Tests made with 
great care, he says, have shown that it is possible to produce 
from natural gas, by a combination of the methods of stage com- 
pression and rectification, not only gasoline of the most excellent 
quality, and equal in every respect to the best grades of refinery 
product, but in addition to recover all of the ethane, propane 
and butane, as a liquid gas, in such a form as to make it a con- 
venient, safe, clean and cheap method of lighting isolated dwell- 
ings, such as country homes, seaside resorts, lighthouses and 
light buoys, etc. 

The name "Gasol" has been given to the liquid gas produced 



222 COMPRESSED AIR PRA CTICE 

by the new process. When under a pressure of 500 lb. to the 
square inch it exists as a clear, transparent Uquid. When the 
pressure is lessened it changes into gas, and at normal or atmos- 
pheric pressure this gas is, unhke the gas produced by blowing 
air through gasoline, for example, extremely dry, and does not 
give any hquid condensate in the pipes, etc., all of the hydrocar- 
bons which so condense having been separated in the process of 
rectification and going into the gasoline product. One volume of 
liquid Gasol produces about 250 volumes of gas upon release of 
pressure, and the gas so produced has a heating value of about 
22,000 calories per liter, or about 2400 B.t.u. per cubic foot. 
When it is remembered that the heating value of ordinary coal 
gas is only about 600 B.t.u. per cubic foot, and manufactured 
oil gas is less than 650 B.t.u. per cubic foot, it will be seen that 
the new gas has about four times the heat-producing capacity, 
when equal volumes are considered, of either coal gas or manufac- 
tured oil gas. In addition, its flame temperature is much higher, 
being decidedly higher than the flame temperature of natural 
gas or any other of the common gases used for heating. The 
flame temperature of ordinary natural gas burning in air is about 
2150°, and the flame temperature of ethane burning in air is 
about 2205°. The flame temperature of the new gas is about 
2300°, and since the amount of light produced from the Welsbach 
mantle bears an important relation to the temperature of the 
flame, the reason is here seen for the remarkable brilliancy of the 
light produced by the new gas, which excels in this respect all 
gases previously known. 

The new process of preparing a liquid gas of perfectly homo- 
geneous nature and uniform composition is the culmination of 
the work of many men, and of several years of experimenting. 
It opens to the use of the world the enormous volumes of oil-well 
gas now so generally wasted, and produces from this waste 
material a product which gives to the country home all of the 
convenience of gas for lighting and cooking, thus giving to the 
farm advantages which have been up to now available in general 
only'^to the city. 



" f^ 


iL 










W |!« \^f 




Kw ^ 


III 


r 


^ . ^^^M 


11 mP 


^ 


., ' !^B 


m^^K^vri 


L..L.^- 




'^^Pte. "flB 


m 


' m 




: '■'^ 




«£* 


m^^- 


^ 




""^MlwH 










>fl 


m " 







CHAPTER XXI 
ROCK-DRILL DEVELOPMENTS 

It would seem that nothing could be more deserving of honor- 
able mention in these pages than the rock drill. While it is so 
entirely dependent upon the air-compressor for the maintenance 
of the most responsible and effective of its activities, nothing 
has done so much retroactively to stimulate the development of 
the air-compressor or to promote the growth of compressed-air 
apparatus in general. 

If we were to ask one only casually acquainted with the rock 
drill to tell us what it is and what it does, we would perhaps learn 
from him that it is one of the simplest of machines, its sole 
function being to reciprocate a piston with considerable rapidity, 
the piston carrying a steel bit which by constant forcible striking 
of the rock at the same spot gradually crumbles its way into it, 
forming a hole of any required depth. This would not be an 
untruthful statement, except as to the first item, but it would be 
an absurdly inadequate one. The simple fact is that there are 
few established inventions which so completely satisfy so many 
imperative and exacting conditions as does the rock drill. 

What is Required of the Rock Drill? — The drill, in a way, does 
rough work, rough surroundings are its habitat, and it is handled 
and operated by men who would also, perhaps unthinkingly, be 
characterized as rough. It must therefore in itself be rough and 
strong exteriorly, so that it can stand the most strenuous use and 
unlimited abuse. It must be able to put in holes in the rock in 
any direction, vertically downward or vertically upward or at any 
angle between, and in any vertical plane all around the circle. 

It must be set in the required position quickly and precisely, 
and while drilling each individual hole it must be held accurately 
and securely, yet never rigidly, in position and direction, and 
provision must be made to constantly feed forward the drill as 
the steel advances into the rock and to withdraw it for changing 
bits or for starting another hole. This suggests the requirements 

223 



224 COMPRESSED AIR PRACTICE 

for the drill mounting, which are entirely separate from those 
of the drill itself and are trivial in comparison. 

Perhaps the most important and responsible detail of the drill 
is its valve and valve motion. There can be no positive valve 
movement as with a steam-engine, for the double reason that the 
stroke of the piston in the drill cylinder constantly varies and that 
if an otherwise satisfying mechanical movement and connections 
could be devised the mechanism would be soon demoralized by 
the jar of the machine; and yet the valve must and does admit 
the air or steam freely and promptly so that the piston moves 
with the required force ; the piston must normally avoid striking 
the cylinder heads, and whatever the position of the drill or of 
the piston in the drill cylinder it must always and instantly obey 
the throttle. The possibility of occasionally striking the cvlinder 
heads seeming to be unavoidable; they cannot be rigidly secured 
to the cylinder but must yield a little upon occasion for their 
own salvation. 

The drill bits, of course, wear in use, and in hard or gritty rock 
the wear may be rapid, so that it must always be possible to 
change a steel for another; the chuck must therefore hold the bit 
securely under the shock of the repeated blows, and yet it must 
be tightened or loosened without delay. This constant changing 
of bits, and the minute movements of the shank in the chuck 
under the repeated blows, of course entails wear, which must be 
provided for, and the chuck or its interior nuist be renewable 
so that the rectilinear truth of the successive bits when inserted 
may be relied on. 

Constantly while the steel is advancing into the rock it must be 
kept in rotation, the turning of piston and steel not having any 
regularity of angular movement, but still insuring that the cutting 
edges of the bit shall each time strike in a different place. The 
automatic rotation provided, while it insists upon the turning 
of the piston and bit more or less for each stroke, still does it in a 
yielding way, so that accidents or breakages do not occur if at 
any time the bit refuses to be immediately turned as far as the 
rotation device would normally require. 

It was conceded at the beginning that the rock drill is in a way 
a rough machine, and yet in the details of it there is scarcely 
any machine so carefully made and so precisely sized in all 
responsible dimensions. The drill, for instance, is, as we know, 
normally driven by compressed air, and the readiness of com- 



ROCK-DRILL DEVELOPMENTS 225 

pressed air to leak away wherever it finds the chance is more than 
sufficiently notorious, and yet the cylinder heads, the valve 
chests and other parts requiring air-tight joints are so perfectly 
surfaced and fitted that they go together, and are always used, 
metal to metal, absolutely without packing. 

The rock drill leads the strenuous life. It is necessarily subject 
to severe wear, much abuse and frequent accidents, so that its 
parts are liable to require renewal or replacement at any time. 
Every part of every standard drill is made so accurately to gages 
that stocks of such parts are maintained wherever the drills 
may be employed in any part of the world, and these pieces 
interchange with and replace each other as though there were but 
one machine of its type in existence. 

The manufacture of the rock drill as it is to-day imperatively 
requires the coincident compliance with many exacting condi- 
tions. After the drill has come to be practically adapted to its 
special line of work by the successive accession of innumerable 
inventions, the trying out of the successful details, the determin- 
ing of the absolute and the relative sizes of the several parts for 
their mutual cooperation, the selection of the most suitable 
and enduring material for each part and its proper heat treatment 
and manipulation to secure strength and slow wearing qualities, 
the system and means of production are to be elaborated. This 
means carefully collected and trained and long experienced 
workmen, installations of automatic and often special machinery, 
costly collections of gages, jigs, templets, drills, taps, reamers, 
of original tools for special operations, and the breaking in, the 
testing and the rigid inspection of the product before it is sent out. 

The rock drill began its life work in tunnel driving, and suit- 
able mountings were provided for it. Now it is used in sizes 
larger and smaller for drilling both below and upon the surface, 
and it is held indiscriminately upon tripod, shaft bar or column, 
upon the sliding and angularly adjustable frame of the gadder 
and in the heavier sizes for subaqueous surface drilling; and it is 
further transformed in the self-contained channeler and the coal- 
puncher. 

These suggest the lines of growth and development of the origi- 
nal percussion drill with reciprocating, bit-carrying piston. It 
may be said to be some GO years old, but in the last score of years, 
even in the last decade, its status and prospects have been rapidly 
changing. 



226 COMPRESSED AIR PRACTICE 

At the very beginning it was ft radicftl departure in principle 
of opei'ation from the hand-drilUng process which it was to super- 
sede. In tlie hitter the drill was held stationary against the rock 
with usuallj' two men striking with sledges upon the head of it, 
there being some hand rotation of the drill after each successive 
blow. The new drill seems to have been such a success that no 
one for the time thought of questioning the correctness or finality 
of its working principle. 

Then came along the "pneumatic tools," so-called, designed 
to take the principal part thereafter in the former exclusively 
hand operations of stone carving, the chipping, caulking, rivet- 
ing of the metal worker, and work of that class in all the trades. 
These tools returned at once to the principle of operation which 
had lived through the ages in hand work, the chisel, or whatever 
the tool might be, being pressed upon the work and the blow being 
showered upon the tool socket. The ** pneumatic tools" were 
such a pronounced success from the start, and have so rapidly 
developed their ada}Ua.bility and extended the scope of their 
actual employment tliat their incursion into the rock-drilling 
field was inevitable, and accordingly we have already lines of 
rock drills actuated upon the pneumatic hanuner principle and 
with pronounced success. These drills have not so much crowded 
the reciprocating piston drills in their legitimate lines of work 
as they have struck out new lines for themselves where the drill 
of the old type was at a disadvantage. 

AVhat immediately follows I mostly abstract from an editorial 
in the Engiuccring and Mining Journal, the writer of which is 
evidently more closely in touch with actual mining practice as 
it is to-day. Three types of machine drills, he says, are now 
well recognized: The piston drill, always mounted, with recipro- 
cating drill-bit rigidly attached to the piston-rod, automatic- 
ally rotated and fed by hand, and with the cuttings removed by 
gravity or by the churning of the bit in water introduced at the 
collar; the hammer drill, always mounted, in which the bit does 
not reciprocate, but is automatically rotated and fed by hand, 
the cuttings being removed by a stream of air or water through 
the steel, the piston striking the shank of the steel a blow similar 
to that delivered by a hand hammer on the head of a nail; and 
the *'jackhtmimer'' or *'plugger" drill, always unmounted, in 
which the bit does not reciprocate but is rotated by the operator 
and is fed by the weight of the machine and the operator, the 



ROCK-DRILL DEVELOPMENTS 227 

cuttings being removed by gravity or by a stream of air or water 
through the steel. This latter is essentially a one-man tool, 
held in the hands, progress being made either through the weight 
of the drill, through the pressure by the operator, or both. 

The hammer drill proper is suitable only for drilling upwardly 
slanting holes, and consequently finds its field of usefulness esp(;- 
cially in stoping, whence the name ''stoper " often applied to these 
drills. The jackhammer, on the contrary, cannot very well be 
used for upward holes on account of the inconvenience to the 
operator, who holds it without support, but as a tool for drilling 
down holes it is marvelous. At present it is being employed in 
shaft sinking at the Newport mine, in Michigan, and we hear 
with highly economical results. The men take these instruments 
with them just as they used to take hand hammer and steel in 
olden days and save the time of rigging and unrigging the heavier 
machine drills, with obvious advantage. So far as we are aware, 
the shaft sinking at the Newport mine is the first use of the jack- 
hammer in such work. 

It is thus brought out that each of these types of drills has its 
special field. In drifting, the piston and the hammer drill divide 
the field, the two-man or the one-man size being required accord- 
ing to the difficulty of the ground. In sinking, the piston drill 
is still pre-eminent, but the hammer drill, and especially the jack- 
hammer, have been rapidly winning favor even for steep down 
holes, and the jackhammer is being successfully used also in 
shaft sinking. In ''raising" the hammer drill or "stoper" is 
at its best, but in work rising at a low angle the piston drill is 
still supreme. In stoping the choice is determined by the nature 
of the deposit. If the deposit is flat or if it is taken off in flat 
benches, then the piston drill is the logical choice. If it is steep, 
comparatively narrow and mined by overhead stoping, the ham- 
mer drill is much superior. If glory-hole methods can be adopted 
the jackhammer will probably be selected. 

In some of the Michigan copper mines the small drills have 
contributed to remarkable results. In these the piston drill of 
the old type has been gradually replaced by a newer and later 
type of piston drill which is partly, if not entirely, a one-man ma- 
chine. Recent figures in bench or footwall work have shown that 
the cost of mining with this light piston drill is about $0.56 per 
ton, each machine giving about 11 1/2 tons per man per shift. 
With the hammer drill the cost per ton in the same, or similar, 



228 COMPRESSED AIR PRACTICE 

material is reduced to about $0.45, each man producing about 
12 1/2 tons per shift. 

In work where the jackhammer can be used to advantage it 
is obvious that a still greater efficiency ought to be obtained, 
because of the saving of time in carrying and mounting the drill. 
In fact, increased efficiency is actually realized, and to a high 
degree, the cost per ton with the jackhammers being about $0.15, 
each man producing about 36 tons per shift. 

In these comparative figures of cost no overhead charges are 
included nor the cost of the air. The former will be different at 
each mine and with each organization and the cost of compressed 
air also is variable, so that comparisons may be made more fairly 
without including these. In a general way it may be assumed 
that it costs from $0.50 to $1.00 for 24 hours per drill for com- 
pressed air. With the jackhammers the cost is reduced by 
approximately one-half as compared with drills of the other types. 

The old type of piston drill weighs about 250 lb., the newer 
and Hghter piston drill about 150 lb. and the jackhammer only 
about 60 lb. Naturally its lighter weight and the usage to which 
it is subjected result in a shorter life, but its first cost is only a 
little more than half of that of the piston drill, and its efficiency 
is so much greater that it is economical to adopt them, wear them 
out, and replace them with new ones as required. 

Some marvelous work in reducing mine costs has been done in 
the Michigan copper country during recent years, and this has 
been achieved largely by the introduction of improved methods 
of drilling and improved drills, in which it must be said that the 
manufacturers have rendered highly valuable assistance to the 
mine engineers. In fact in mining machinery as in other things 
the principal inventions have originated outside of the mines and 
the inventors and designers have not been miners; although 
necessarily they have been more or less informed as to mining 
possibilities and requirements. 

The progress in mining drill practice and the increase in general 
efficiency is after all not to be credited to the drills alone. There 
have been improvements apart from the drill itself which have 
directly or indirectly helped to augment the results accomplished 
by the drill; that is in the records of cubic yards of rock broken 
or of tons of ore got out. 

For one thing, the modern high explosives should have due 
credit. The enormous advance in the effects produced has made 




Fig. 67a. — Typical Stunt with a " Jackhamer" Drill in a Montana Copper 

Mine. 

{To Face Page 228.) 



ROCK-DRILL DEVELOPMENTS 



229 



it unnecessary to drill so large holes in the rock, and this has 
helped to provide the opportunity for the light hammer drill, or 
for the one man drill in general. With the old black powder 
and the holes which it would have required, both in size and num- 
ber, to produce the same results the case would have been 
different. 

In Figs. 68 and 69 we have a rather curious illustration of the 
difference in the force and effect of the old explosives and of the 




Fig. 68.— Effect of Black Powder. 



Fig. 69.— Effect of Dynamite. 



new. The first shows the bottom of a hand drilled hole on the 
line of the original Croton aqueduct located about half a mile 
below High Bridge, and the other, not a quarter of a mile from the 
first, shows the bottom of a machine-drilled hole on the side of 
a street only recently cut through the rock. In the first case 
the outline of the hole is still perfect, showing that the powder 
used had not force enough to shatter the rock at all. In the sec- 
ond case where dynamite, or some otherwise named modern 



230 COMPRESSED AIR PRACTICE 

explosive, was used the force was so intense and concentrated that 
the rock immediately surrounding the charge was so shattered 
as to leave no trace of the shape of the drilled hole for a couple 
of feet of what was evidently the lower end of it. 

The Drill Sharpener. — But another thing which is helping 
the drills to accomplish more and to estabUsh new records is in 
the better maintenance of the shape, size and condition of the 
cutting ends of the drill bits. The hand. dressing of rock drill 
steels has always been so unsatisfactory that it would seem that 
the invention of a machine to do the work was inevitable. The 
lateness of its arrival showed that the devising of such a machine 
was no trivial task for the inventor, and the perfect success of 
the latest machine speaks for itself of the inventive skill and the 
thoroughness of adaptation of means to ends in the work of the 
designer. 

The machine is, first of all, as it necessarily must be, more 
properly a drill maker than simply a drill sharpener, since it 
takes the end of the original bar of steel and upsets it and shapes 
and sizes it for its first work, also forming the shank on the other 
end, and the simpler job of '^sharpening" the steel, or of restoring 
its shape and size comes later. 

The machine is pneumatically operated throughout, using the 
air pressure provided for running the drills. The end of the steel 
is heated as hot as is permissible and then, in a horizontal posi- 
tion and regardless of the length, it is placed in the machine and 
is tightly gripped by a vertical movement of the upper portion 
of the vise. The jaws which grip the steel also form, when closed, 
a conical die for shaping the outside of the bit. Immediately 
that the steel is gripped a '' dolly" is advanced and pressed against 
the end of the hot steel and simultaneously with this movement 
a pneumatic hammer action showers rapid and forceful blows 
behind the dolly and the steel is quickly upset to fill the die and 
the cutting face of the bit is shaped at the same time. The* 
entire action of the machine is controlled by a single lever and 
the entire operation is one to be watched with pleasure by any 
one endowed with a modicum of mechanical instinct and sympathy. 

The advent of such an efficient machine as this changes the 
attitude of the drill runner at once. It is no longer necessary 
for him to be thinking of saving blacksmith work or to continue 
to use steels after they lose their gage or have ceased to cut at 
their best. Not only do the steels cut more rapidly when working 



ROCK-DRILL DEVELOPMENTS 231 

but they follow each other better and there is less difference in 
gage between the starter and the bottomer. The more fre- 
quent changing of the steels which the mechanical drill sharpener 
encourages is an important time saver rather than the reverse, 
and where more than two or three drills are working together 
the machine should soon pay for itself. 



17 



CHAPTER XXII 
THE ELECTRIC AIR DRILL 

It is generally the deliberate purpose of the present volume not 
to describe in detail individual devices which have been developed 
and have found successful employment in compressed-air prac- 
tice. The electric air drill, however, is in a class by itself, and 
may claim mention somewhat anomalously from the fact that, 
while it is an alert and winning rival of apparatus which belongs 
entirely to the compressed-air class it cannot properly be included, 
in any precise enumeration of compressed-air devices. It ig- 
nores absolutely the standard air compressor and the pipe line. 
It is in fact also not at all an electric drill, and might as well be 
driven, if equally convenient, by a belt or by a steam-engine 
direct. The name which has been given it, intended to be de- 
scriptive of it, is not at all satisfactory, and is apt to be misleading 
without accompanying explanations. 

The application of the electric current to the direct and 
immediate driving of the percussion rock drill has been an increas- 
ingly urgent problem for a score of years, but there has been little 
reason at any time to hope for a satisfactory solution. With 
the advent of the electric air drill the problem is not indeed 
solved, but it is annihilated. There is a better way, as this drill 
shows us, to apply the electric current to rock drilling than by 
means of any bona fide electric drill. This drill, in its ultimate 
actuating element is actually an air drill rather than an electric 
drill, using the electric drive, indeed, for the motor detail of it in 
the most advantageous way possible, without any of the disad- 
vantages inseparable from the electric drill as such, with all the 
advantages of the best compressed air driven drill, with special 
additional valuable features of its own, and without any ace om- 
panying disadvantage. 

These are large claims, but an understanding of the drill and its 
mode of operation would seem to justify them. Fig. 70 shows an 
electric air drill as used in actual work with all the apparatus per- 
taining to it. The drill itself looks quite like a standard com- 

232 




a 

o 

o 



THE ELECTRIC AIR DRILL 233 

pressed air driven drill, and it may be mounted upon tripod, bar 
or column in the usual way. There is the usual drill cylinder and 
piston, the piston rod carrying the bit or steel which does the 
actual rock cutting. The cyHnder sUdes in a shell, moved back 
and forth therein by the usual hand operated feed screw. Near 
each and of the cylinder there is a projecting boss for attaching 
a hose, with free, direct openings into the cylinder. The piston 
is short and the piston rod, which runs in a long sleeve extension 
of the cylinder, is much larger than in the usual air drill, so that 
no enlargement of the piston rod for the chuck portion is required. 
The chuck used, which is an incidental and independent invention, 
is a special self- tightening device fitting the taper end of the rod. 

The little truck, one of which accompanies each drill and is 
an integral part of the complete apparatus, has a small electric 
motor, either direct or alternating current, driving by reducing 
gears a shaft with a crank on each end which cranks give alter- 
nating movement to two vertical, single acting trunk pistons in 
corresponding air- cylinders. One of these cyhnders is connected 
by a short length of hose to one end of the drill cylinder, and the 
other end of the drill cylinder is connected by a similar hose to 
the other air-cyhnder. The body of air thus enclosed in each 
air- cylinder, in its hose and in the connected end of the drill 
cylinder is never discharged or changed, but plays back and forth 
as the piston moves. The air-cylinders are thus not compressors 
but pulsators, as their function is not only to furnish effective 
driving pressure to one side of the drill piston but alternately 
also to reduce the air resistance at the other side of the piston. 

The principle of operation is simple and easily understood. 
The entire apparatus, both of the hose, both pulsator cylinders 
and both ends of the drill cylinder, in normal working condition 
is filled with air at about 30 lb. pressure. The correct supply of 
air is accumulated and maintained automatically. If through 
leakage or otherwise the charge of air becomes insufficient there 
occurs in the cycle of operation a moment when the pressure of 
the air becomes less than that of the atmosphere, and at once an 
inwardly-opening check valve admits sufficient air to supply the 
deficiency. This arrangement also operates to charge the system 
with sufficient air when first starting up. 

The air being then at normal pressure, as the cranks revolve 
the piston in one of the pulsator cylinders advances and the pres- 
sure is increased in the connected end of the drill cylinder. At 



234 COMPRESSED AIR PRACTICE 

the same time the pressure in the other air- cy Under and at the 
other end of the drill cylinder is reduced, this difference of pressure 
causing the piston to make its stroke. As the crank shaft con- 
tinues to turn the movements of the pulsator pistons are reversed, 
the pressures on the two ends of the drill piston also are reversed, 
the drill piston makes its stroke in the opposite direction, and 
so on continuously. 

This arrangement results first of all in a great simplification 
of the drill and its appurtenances and a lowering of the total cost 
of maintenance. The usually most expensive and troublesome 
parts of the compressed-air drill are got rid of without any sub- 
stitutes or equivalents in place of them. The drill has no valve 
of any kind and no valve operating devices, no tortuous air ports 
or minute air-valve passages, no split front head, no buffers or 
yielding head fastenings, no springs or side rods, not even a 
throttle valve. The parts thus discarded are those which are 
always giving out or making trouble on the compressed-air drill, 
and are the most expensive details to be maintained. The pul- 
sator cyhnders also, which entirely supersede the compressor 
plant, have no valves, either inlet or discharge, and not even a 
water jacket, as the cylinders have no tendency to heat up when 
working. The apparatus here spoken of, the drill and pulsator, 
it is to be remembered, comprises everything beyond the wires 
which connect with th^ generator at the power house. 

The changes in operation and in the results accomplished by 
this novel apparatus are well worth looking into, several surpris- 
ing results appearing. The electric air drill, it is claimed, 
strikes a harder and a livelier blow than that of the standard 
air drill. To this effect several things contribute. The piston 
is air cushioned at each end of the stroke, which the other is not, 
so that it starts its next stroke impelled by a certain amount of 
force saved and brought over from the preceding stroke. Then 
when it begins to move there is a simultaneous reduction of pressure 
in advance of the piston and an increase of pressure behind it, 
both conditions the reverse of those of the compressed-air drill. 
With the old type of drill, driven by air at constant pressure, 
which the drill piston runs away from, there must necessarily be 
some fall of pressure as the speed of the piston increases, and at 
the same time there must be, and for the same reason, an increase 
of opposing pressure in front of the piston, and a decreasing 
effective difference between these operating pressures. With 



THE ELECTRIC AIR DRILL 235 

the electric air drill the pulsator piston may be said to be chasing 
and gaining on the drill piston on the driving side, thus increasing 
the driving pressure, at least up to mid-stroke, while the other 
pulsator piston is running away from the other side of the drill 
piston and reducing the pressure in front of it. 

It would seem that the critical point of the stroke of this drill, 
the point in the direction of the piston movement at which the 
actuating force culminates for the delivery of the most effective 
blow upon the rock, must be more precisely located in this drill 
than in the familiar compressed-air type, where all the runner has 
to look out for is to come as close as possible to the front head 
without hitting it too often; but if this be true the operator soon 
comes to know by instinct the point of best efficiency and feeds 
accordingly. 

Perhaps the most astonishing result shown by the electric air 
drill is in the reduction in the amount of power required to ope- 
rate it. It has been practically demonstrated that the power used, 
at its source in the power house generator, is only about one- third 
of that required by the compressor which supplies the compressed 
air drill of the same capacity. A 5-h.p. motor drives the electric 
air drill equivalent in capacity to a 3 1/4 compressed air drill, 
the latter when most effective requiring 20 or more horse-power 
at the compressor. A concise explanation is that the same air is 
used over and over again, its expansive force is fully employed, 
there are no thermal losses, there is no filling and empying 
of large clearance spaces. 

The pulsator motor runs at constant speed, there t)eing two 
different speeds available when alternating current is used and 
four different speeds with direct current. The drill piston makes 
a stroke for each rotation of the crank shaft, or at least the full 
air column impulse is transmitted to it. When the drill sticks 
in the hole the pulsator does not stop, or even hesitate, but keeps 
on delivering its alternate thrusts and pulls at the regular speed, 
say, 400 per minute, and nothing can well be imagined more ef- 
fective for yanking the bit free again, and when it is released the 
strokes are resumed without any stopping and starting delays. 

Provision is made for supplying oil to the drill as it may be 
required. The air carries the oil to every part, and the lubricant 
as well as the air is used over and over, and with a full mechanical 
guarantee, as we might say, that every working part will get its 
share. 



236 COMPRESSED AIR PRACTICE 

There can be no freezing up of the electric air drill, for the 
double reason that it does not accumulate moisture precipitated 
from successive charges of saturated compressed air, and also 
because the working temperature of the air in the drill system 
never falls low enough, both high and low temperatures being 
avoided by the special conditions of air employment. This non- 
freezing feature makes the electric air drill entirely independent 
of climatic conditions, drills of this type being run all winter in 
quarry or other exposed work as well as in summer. 

The drill being thus independent of climate is also quite as in- 
dependent of altitude. As the air is used in a closed circuit it 
makes no difference to it what the external atmospheric pressure 
may be, and in the deepest mines or on the highest mountains 
accessible to man, wherever the electric wires can reach it, it 
runs just the same. 

If there is any special line of work to which the electric air 
principle of drill operating is better adapted than any other it 
would seem to be for rock channeling. In the standard or ac- 
cepted rock chamber a large air cylinder is employed, normally 
vertical but permitting of working to a considerable angle, and 
to the piston rod is suitably attached a holder which carries a 
gang of steels, and while the piston is reciprocating and the steels 
are striking the rock, the entire machine is slowly moved back and 
forth a determined distance, thus cutting in the rock channels of 
considerable length and to depths of 5 or 6 ft. or more. This work 
naturally requires much more power than a single drill steel and 
the piston in a channeler of this type being normally steam driven, 
the entire apparatus, a boiler and all appurtenances, is mounted 
upon a heavy truck traveling upon a temporary railway. Dis- 
pensing with the boiler the truck provides the ready means for 
carrying the pulsator apparatus, making the electric air channeler 
entirely self-contained and dispensing with the little carriage 
which otherwise is the inevitable accompaniment of the electric 
air drill. These electric air channelers are in use to the number 
of a score or more in a single quarry. 

Electric Air Drills at Kensico Dam. — Perhaps the most notable 
job which the electric air drill has found up to the present writing 
is in connection with the construction of the Kensico reservoir 
of the Catskill aqueduct system for the water supply of New York 
City. The reservoir will have a shore line of over 30 miles with 
a storage capacity of 40,000,000,000 gal., and the dam, it is claimed 



- 



THE ELECTRIC AIR DRILL 



237 



will be the largest in the world, containing over 1,000,000 cu. yd. 
of masonry. For the construction of this dam and the related 
reservoir work an equipment was provided of 30 electric air drills 
of standard type and four of the electric air drills of much greater 
capacity, each mounted upon a drill wagon similar in many re- 
spects to the electric air channelers previously spoken of. 

The general direction of the dam is northwest and southeast, 
and about half a mile from the southeastern end a site was 
selected for a quarry which would yield clean, solid rock, and from 
this the material for the dam was to be procured. The original 
surface of this quarry site was covered with second growth 
timber which was cut off, and when the soil had been removed 





1 -I- 


^Mll 


■t^ 


\^^^4 


SS 


* 





Fig. 71. — Electric Air Drills at Quarry for Kensico Dam. 



the glacier-scored surface was ready for the drills. No time was 
lost here, and Fig. 71 shows a group of electric air drills put- 
ting down the necessary holes. More than 1200 of these holes 
were fired at once. The material was then handled by steam 
shovels and dumping cars which brought it to the giant rock 
crushers. 

The tripod drills put down holes to depths of 15 to 20 ft., while 
the drill wagon worked where greater depths were required to 
have the holes bottom near a common level. 

These drills were not adopted at all as an experiment, but upon 
reliable evidence in advance as to what they could do, and they 
did it. The electric air drill complete costs more than the air- 
driven drill, but as the latter with its share of the compressor which 



238 COMPRESSED AIR PRACTICE 

drives it, and the piping and appurtenances, costs much more than 
the electric air drill, and involves far more first cost when instalhng 
a plant; the latter shows a great advantage. The piping and 
valves and other requirements of the air drill will more than 
cover the excess of cost of the electric air drill, and then we might 
say that there is the cost of the big air-compressor plant on the 
one side and that of the electric generator upon the other, but in 
so many cases, like the present, the generating plant costs noth- 
ing, because current can be furnished by the big electric com- 
panies cheaper than it could be produced by any isolated plant. 

Power Cost of Electric Air Drilling. — But the most interesting 
and really the most important question in regard to work of this 
extensive character is as to the power consumption and the total 
cost of operating. Here we have results which can only be 
characterized as astonishing. 

First as to the drill wagon. This drilled holes to an average 
depth of 30 ft. using a 5 to 5 1/2 in. starter and bottoming at 4 
to 4 1/2 in., the aggregate depth of holes drilled for 8 hours 
ranging from 45 to 65 ft. Under test conditions 104 ft. of hole has 
been drilled per shift. Under the best conditions the cost with 
another standard type of drill would average between $0.80 and 
$1.00 per foot. The actual cost per foot on the electric air drill 
wagon is made up as follows: 

Electric power for 8 hours $0.60 

Drill runner 4.00 

Helper 2.50 

Basing the cost per foot upon the power and labor charges 
alone, and considering the average daily work as 50 ft., which is 
very conservative, would bring the cost per foot to approximately 
$0.14. With a driller representing the builders of the drill this 
cost was cut in half. 

This drill wagon is as readily handled and operated as anj^ of 
the class of drills with which it directly competes, and its cost of 
maintenance is believed to be no greater, but the extraordinary 
feature is the power cost. This sometimes runs as low as $0.30 
per day, and is never higher than $0.75. It is understood that 
the electric current in this case — 3 phase, 60 cycle, 220 volt — 
cost the contrator $0.0125 (1 1/4 cents) per kilowatt-hour. 

The power cost of the electric-air tripod drills is between 30 
and 40 cents per day, drill runner $3.50 to $3.75 per day and 



THE ELECTRIC AIR DRILL 239 

helper $2.50. The holes are 10 to 15 ft. deep, bottoming at 
1 3/4 in. diameter, and the drilling per shift is 35 to 45 ft. This 
would make the drilling cost of these machines based on power 
and labor (power 35 cents per day) approximately 20 cents per 
foot of hole drilled. 

Comparison is suggested between the performance of these 
electric-air tripod drills and the standard 3 1/2-in. air operated 
drills. Assuming that the average free air consumption per drill 
per minute is 150 cu. ft., compressed to 90 or 100 lb., and taking 
the air from the highest type of Corliss compound condensing 
engine-driven compressor, with high pressure boiler, etc., 
would bring the power cost alone to 75 cents per day. Adding 
interest charges on the entire compressor installation, cost of 
appurtenances, pipe lines, etc., maintenance and depreciation, 
would bring this cost of power per drill per day to $1.25 or $1.50, 
or between three and four times the power cost shown for the 
electric air drill. 

There were also working upon this contract some 3 1/2 in. 
air-driven drills of the type referred to above, which began work 
before the electric air drills were installed, and it has been found 
that with both drills working in the same rock the electric air 
drills actually averaged from 5 to 10 ft. per shift more than the 
air drills. 

If instead of assuming the most economical type of compressor 
for the above comparison the air had been taken from a straight- 
line compressor of the old and still familiar pattern, working 
non-condensing, with steam below 100 lb., etc., the discrepancy 
in costs would be much greater. In these installations experience 
has made it common practice to figure the power cost per drill at 
$2.50 to $3.00 per day. 



CHAPTER XXIII 
COMPRESSED AIR FOR RAISING WATER 

One of the most obvious uses of compressed air is for raising 
and conveying water and other liquids, and this has become one 
of its most extensive fields of employment. The variety of 
ways in which the air is applied for this purpose and the diver- 
sity of the apparatus that has been devised are astonishing. 
Many have nothing more than a historic interest; the actual, 
practical ways in which air is now employed for pumping, while 
differing widely from each other in efficiency and other partic- 
ulars, are not numerous, yet they would be none the worse for 
still further elimination. The conditions under which the water 
is to be raised largely determine the specific device employed in 
the individual case, but sometimes other considerations not so 
defensible prevail in the selection or in the retention of deservedly 
obsolescent systems. 

There is, according to the system adopted, much difference in 
the amount of air consumed as compared with the work done, 
but in all cases the former must be in excess of the theoretical 
requirement, as nothing can be done for nothing. With our pres- 
ent knowledge, it still pays in many cases to use air for raising 
water, in both small and large quantities, and as a means of 
permanent supply as well as in temporary or emergent cases. 

It is desirable not only to do, but also to know that we are doing, 
the work as cheaply as possible. Our present facilities make the 
metering of water lifted or transferred an easy thing to do, and 
the raising of water, by compressed air or otherwise, can always 
be gaged with satisfactory accuracy, while records of such work 
are constantly accumulating and are accessible as guides to the 
engineer. 

The theoretical horse-power required for raising water is : 

Pounds of water per min. X height of lift in ft. 

Table XX may be taken as a starter; it furnishes the essential 
data as to the power required for raising water to different heights. 

240 



i 



COMPRESSED AIR FOR RAISING WATER 241 



TABLE XX.— STATIC WATER POTENTIALS 



1 


2 


3 


4 

Potential 
h.p. in 


1 


2 


3 


4 

Potential 

h.p. in 


Gal. 


Volume, 


Weight, 


water 


Gal. 


Volume, 


Weight, 


water 


per min. 


cu. ft. 


lb. 


raised 
100 ft. 


1 


cu. ft. 


lb. 


raised 
100 ft. 


1.0 


0.13368 


8. 355'0. 025303 


200 


26.736 


1671 


5.0606 


2.0 


0.26736 


16.710 0.050606 


250 


33.420 


2089 


6.3257 


3.0 


0.40104 


25.065 


0.075909 


300 


40.104 


2506 


7.5909 


4.0 


0.53472 


33.420 


0.101212 


350 


46.788 


2924 


8.8560 


5.0 


0.66840 


41.775 


0.126515 

1 


400 


53.472 


3342 


10.1212 


6.0 


0.80208 


50.130 


0.151818 


450 


60.156 


3760 


11.3863 


7.0 


0.93576 


58.485 


0.177121 


500 


66.840 


4177 


12.6515 


7.48 


1.0 


62.5 


0.18928 


550 


73.524 


4595 


13.9166 


8.0 


1.06944 


66.840 


0.202424! 


600 


80.208 


5013 


15.1818 


9.0 


1.20312 


75.195 


0.227727 


650 


86.892 


5431 


16.4469 


10.0 


1.3368 


83.55 


0.25303 


700 


93.576 


5848 


17.7121 


20.0 


2.6736 


167.10 


0.50606 


750 


100.260 


6266 


18.9772 


25.0 


3.342 


208.87 


0.63257 


800 


106.944 


6684 


20.2424 


50.0 


6.684 


417.75 


1.26515 


850 


113.628 


7102 


21 . 5075 


75.0 


10.026 


626.62 


1.89771 


900 


120.312 


7519 


22.7727 


100.0 


13.368 


835.50 


2.5303 


950 


126.996 


7937 


24.0378 


150.0 


20.052 


1253.0 


3.7954 ; 


1000 


133.680 


8355 


25.303 



It gives also the actual potential energy in the water so elevated, 
or the power which it should be theoretically possible for the 
water to develop in its descent to normal level if employed in a 
water-wheel or motor. When the power actually consumed in 
a water-raising operation is ascertained it can be compared with 
this table and the result will be an indication of the efficiency in 
the given example. 

The first column gives the number of gallons of water lifted; 
column 2 gives the volume in cubic feet of the given number of 
gallons, while column 3 gives the weight in pounds of the same 
quantity of water. Column 4, assuming that the given quantity 
of water — gallons, cubic feet or pounds — is raised to a height of 
100 ft. in a minute, gives the horse-power theoretically required 
for the lift, or the horse-power which should be developed by the 
descent of the water to its original level. Any other figures or 
quantities not in the table will be in direct proportion to those 
given. 



242 



COMPRESSED AIR PRACTICE 



The diagram Fig. 72 is based upon the same data as Table XX, 
but reaches farther, both as to volume of water and height of 
lift. 



^ ^T 


\ J 


>^ ^ u 


^ A t 


V \ A 


^ X J 


V -^ W 


N t i 


V ^ A A 


^ V \ ^ - ^ 


\ ^ ^ c 


V ^ A I 


^ ^ V 4 


dl V ^^ A ^ 


^ ^^ ^ ^ f 


s V ^> r TT 


S ^v X A T^ 


^ \ A ^^ w * 


\ v \^ A W~ 


^\^^ V V V Vl- 


^^^^^ A A r ji 


^^^^x \ V t W 


^ ^^ \ v^ "^V t 


^ ^^ V % K X A 


^ \ ^ \% VA \ J 


^^ \ ^s^s ^^v rzr 


^i. ^;^^v\ ^ \ A 4 


^ xti^vCv vX\a 


"^^ ^^^^'n^O^A \ X^ 


^ \;^X^x^^5i^^A r 


>, ^. \ VWX \H \ \ 


~-^\x^A>\^X\A 


-^ ^^ ^5.4X\Vv s^O^ 


~~-- ^ ^\,3S^^Xvr 


^ ^^ V\NN^s\\\\\ 


-----, ^^"^-.^^SlsSA 


^^^ ~---.^^^^!v 


■~--L-.---.>^:<;m\\ 


"'■ — ' — r--r ■H^:^::^^^ 


1 — r-P^^^^^' 



s s 



S 8 



i s I i I I I 



8 g 8 

A « Ci 



8 S 









Driving Steam Pumps with Air. — First of all (as an example of 
''how not to do it") we may consider what it costs to drive an 
ordinary, direct-acting steam pump by compressed air. It is 
often so convenient to do this, in mines, tunnels, excavations for 
foundations and elsewhere, that it is done generally without 
counting the cost, either before or after. 



COMPRESSED AIR FOR RAISING WATER 243 

The simplest case is where the steam or air and the water 
cylinders are of the same diameter, and have the same stroke. 
Here if there were no allowances to be made for clearance losses, 
leakages, power required to overcome friction and inertia, and 
if there had been no losses involved in compressing and trans- 
mitting the air — all these and other things being those which pure 
theory is so apt to beUttle or ignore — the volume of air at the 
balancing pressure would just equal the volume of water deUv- 
ered, and the efficiency would be 100 per cent. 

The standards of efficiency which we call the possible efficien- 
cies are really the impossible efficiencies. It is well known that 
they can never be attained, and very far from it in the present 
case. The air pressure must be enough in excess of the water 
pressure to overcome the frictional resistance of the machine 
itself, and of the water in contact with the surfaces in its restricted 
flow through valves and passages, and to give and maintain 
sufficient impulse in the otherwise inert column. For all this 
it will be proper to allow an initial deficiency in the air power at 
the pump which will average not less than 20 per cent., and the 
100 per cent, efficiency with which we have started is reduced to 
80 per cent. 

But if there is such a deficiency, the pump will not go. This is 
understood; and the several deficiencies are anticipated and pro- 
vided for beforehand by furnishing air in sufficient volume and 
at an excess of pressure to overcome them. 

To run a pump comfortably the working pressure of the motor 
fluid should be somewhat above the actual requirement, but it 
is not necessary to speak of that here as affecting the power 
consumption. 

Next, the matter of cyfinder clearance may be considered, which 
is interesting in the direct-acting steam pump at any time, and 
especially so when the pump is driven by air. Nothing need be 
said about clearance losses in the water cylinder, for practically 
there are none, as all the spaces may usually be assumed to be 
filled solidly with water, and if everything is in good order all 
of the actual travel of the water piston is represented by water 
delivered. 

Enough clearance losses in the air cylinder may be found to 
satisfy for both. The direct-acting pump has no crank to bring 
the piston to a dead stop always at the same point at the end of 
the stroke, and, as at the same time it must be certain that the 



244 COMPRESSED AIR PRACTICE 

piston shall never strike the head, very large clearance is pro- 
vided. The filling of this large clearance, together with that of 
the unavoidable clearance spaces in the passages between the 
valves and the cylinder, entails another large excess of air over 
that theoretically required, or a deficiency of work done as com- 
pared with the air consumed of, say, another 20 per cent. This 
percentage of 80 being 16, 64 per cent, of the original 100 at this 
point is left. 

There is a third great loss of efficiency when compressed air is 
used to drive a direct-acting steam pump, because the air is used 
at full pressure and shows none of the advantages of being used 
expansively. If, instead of using the air at its highest pressure 
to fill the cylinder to the very end of the pumping stroke, it could 
have been used in a crank-and-flywheel pump, or in one of any 
other design in which the air could have been cut off at the proper 
point of the stroke, so that it would have been discharged at a 
pressure nearly that of the atmosphere, then the work done for 
the quantity of air used would have been, on the average, varying 
with the initial pressure of the air, say, 50 per cent, more than with- 
out the cutoff and expansion. That is, it could and would have 
done one-half more work, and the failure to do this amounts to 
another deficiency of one-third, or, say, 33 per cent. This per- 
centage of 64 being 21, there is left only 43 per cent, of the original 
100, and this diminution of efficiency is all realized after the air 
has arrived at the pump, and without looking to the losses which 
have accumulated upon it previous to its arrival on the job. 

Not to be Charged to the Air. — Now, the curious thing is that 
not one of the losses above mentioned, nor any portion of any 
of them, is in any way chargeable to compressed air. They all 
inhere in the apparatus and in the system by which the air is 
applied to the work of lifting the water. 

One loss peculiar to steam and entirely absent with air is the 
loss by condensation, concerning which it is not necessary to 
present any figures. It is plain that the air should not be blamed 
for any of the losses of which it becomes merely the agent when 
driving the sieam pump. 

Thus far the direct-acting steam pump alone, and its deficiencies 
or those which it entails, have been considered. Of course, the 
pump has nothing to do with the friction losses in transmission 
from the compressor to the pump. For all such losses and for 
all possible leakages allow, say, 5 per cent., deducting which from 



COMPRESSED AIR FOR RAISING WATER 245 

the 43 per cent., found above, leaves 41 per cent., all the rest 
having disappeared after the air left the compressor. The trans- 
mission loss just allowed would be greater with steam, while some 
of the distances which air may be carried are prohibitive with 
steam. 

While the cost of compressing the air used is not considered, 
attention might be called to the approximate figures for compress- 
ing free air to, say, 80 lb., gage. Assuming a steam-driven, 
reciprocating air compressor and both compressor and pump to 
run regularly under their respective rated loads, the power which 
is being developed in the steam cylinder may be taken as a starting 
point or the basis of efficiencies. First must be deducted a suffi- 
cient allowance for the friction of the entire machine, and for the 
leakage and clearance and other air-cylinder losses, amounting 
altogether to an inefficiency of at least 20 per cent., leaving 80 
per cent. Then in the compression of the air the excess of power 
required for the actual adiabatic compression, instead of the 
theoretical isothermal compression, will be 34 per cent. This 
percentage of 80 being 27, the surviving efficiency will be 53 per 
cent. This being the efficiency of the compressor and 41 per cent, 
the pump efficiency, the ultimate efficiency of the combination 
is 21.73 per cent.; that is, it will take about 5 h.p. in the steam 
cylinder of the compressor to realize 1 h.p. in the actual lifting 
and delivery of the water. Actual results are seldom any better, 
and often much worse, than this. 

At least one hint may be taken from these figures, which is 
that the loss is the least where the working air pressure is the 
lowest. In the case of the direct-acting pump the difference may 
be made in the relative capacities of the air (driving) and the 
water (driven) cylinders, and, in general, the larger the former 
is, the better. Thus supposing the air to be used at 40 lb., gage, 
instead of 80, the excess of mean effective resistance in the air 
compressing cylinder in the act of adiabatic compression would 
be only 22 per cent, instead of 34, and the loss in the air cylinder 
of the pump through not using the air expansively, would be 
relatively the same, so that there would be a saving at both 
ends. 

Direct-displacement Pumps. — Naturally, the first arrangement 
thought of when it is proposed to use compressed air for raising 
water is that in which the water is displaced volume for volume by 
the air, this air being compressed to a pressure corresponding to 

18 



246 



COMPRESSED AIR PRACTICE 



To Compressor 



the height of the lift, and this scheme has been worked up in a 
great variety of ways, but with httle variation of result. 

Fig. 73 shows diagrammatically a pumping apparatus of this 
type. The submerged chamber here shown is assumed to have 
been filled with water and this water is now being expelled by 
air pressure. The air comes direct from the compressor, passes 
the three-way valve and enters the top of the chamber, its pressure 
driving the water up the vertical pipe at the right. When the 
water is all expelled, the three-way valve is changed to the other 
position, shutting off the compressed air and 
opening communication between the interior of 
the chamber and the atmosphere and allowing 
the air to escape. Then the valve in the 
water pipe at the upper right-hand corner of 
the submerged chamber closes by its own 
weight and the pressure of the water above it, 
and the water rushes into the chamber through 
the valve at the bottom. A reversal of the 
three-way valve to the position shown puts the 
compressed air at work again and the con- 
tent of the chamber is expelled as before, and 
so on. The three-way valve is usually actu- 
ated by a float, making the operation auto- 
matic. There are generally two of these dis- 
placement chambers which work in connection, the same valve- 
operating mechanism serving for both, and thus as one is filling 
while the other is discharging, the water flow is almost continuous. 
It is well understood that the displacement pump does not use 
the air to good advantage, but for economy it is still much better 
than the air-driven, direct-acting pump. There is no friction 
of reciprocating parts and the clearance losses are very small, 
so that instead of the 20 and 20 per cent., which is allowed for 
the direct-acting pump, a single 10 per cent, will fully cover these 
items, leaving at this stage 90 per cent, instead of 64. The loss 
for the same pressures in not using the air expansively would, of 
course, be unchanged, and therefore the 34 per cent, which was 
assumed would still apply, and 34 per cent, of 90 being 30 there 
is 60 per cent, efficiency for the displacement pump instead of 
the 43 per cent, of the air-driven, direct-acting pump, and 57 per 
cent, instead of 41 per cent, when the air leaves the compressor. 
The economy of the displacement pump is much better where 



Direct 



Fig. 73 
Displacement 



COMPRESSED AIR FOR RAISING WATER 247 



-,. Compressor 
J_ Intake 



the lift is small, which makes it specially applicable for trans- 
ferring sewage and similar service, especially as the water may be 
loaded with a large proportion of soUds and semi-sohds without 
impairing its action. 

Retum-air Pumping. — The progress of invention in any line 
seems to be one of gradual but continuous revelation and devel- 
opment. Every device, so that it will work at all, seems to have 
a right to be completely tried out, and to have all its possibilities 
and its defects revealed before it can 
expect to be and actually is displaced 
by something better. 

When the power-wasting and other- 
wise objectionable direct air-pressure, 
water-raising apparatus has had its 
every chance and has been found 
wanting, then, and not until then, 
comes another device which is at once 
hailed as just what is needed. The 
principle of it is at once self-evident, 
and its perfect success when applied 
requires no practical demonstration. 

This is the return-air system of com- 
pressed-air pumping. Its operation, 
in contrast to that of the direct-dis- 
placement pump is that while the old 
pump will take all the air that is given ' ^^ 

to it for raising and delivering a given weight of water, and will 
return absolutely nothing of what it receives, this new device 
will take only the pressure and volume of air which exactly pays 
for the work done, and, so to speak, it returns all the change. 

The whole thing can be simply explained, the diagrammatic 
sketch in Fig. 74 perhaps assisting. It must be premised that a 
return-air system is not a cheap and handy little device ready to 
go, for instance, wherever a steam pump might be employed, 
and to do the same work ; it is intended for large and permanent 
installations, each plant being complete in itself and doing no 
other work. 

The Apparatus Used. — There is, first of all, an air compressor 
whose entire business it is to operate the one pump, tlio capacities 
of the pump and compressor being adapted to each other. There 
is nothing special about the compressor except that it is a single- 




FlG 



74. — Return 
Pumping. 



Air 



248 COMPRESSED AIR PRACTICE 

stage, reciprocating machine, and it, of course, may be driven 
by any available style of power transmission and application. 

The pumping part of the apparatus consists of two similar 
chambers to be alternately filled with water and emptied by the 
expulsion of the water to the level required. These chambers 
are submerged in the water to be pumped, or at least are located 
near and below the lowest working level of it, so that the water 
will always be able to flow into either chamber by gravity. The 
compressed air is conveyed to the pump from the compressor 
by separate pipes to each water chamber, and each pipe also leads 
the air, after its work of water expulsion is done, back to the air 
intake of the compressor. It is thus a closed system, and the 
same air is used over and over again, provision being made for 
continually replacing the little that may be lost by leakage or 
by absorption in the water. The valve arrangements are clearly 
shown in Fig. 74. 

Operation. — Assume that the pump is actually at work, that 
one of the chambers is full of water, with sufficient air under pres- 
sure flowing in at the top of the chamber, exerting its force upon 
the water and driving it through a retaining valve and up a water 
pipe to the point of delivery. 

Thus far the operation is precisely the same as with any dis- 
placement pump, and when the water is all, or nearly all, driven 
out the chamber is full of air at the pressure required and deter- 
mined by the height of the water delivery. The regular opera- 
tion by the old way would now be to close the communication 
with the compressor and to open another to the atmosphere, 
allowing the air, expanding as it goes, to escape without doing 
any more work and leaving the chamber filled with air at normal 
atmospheric pressure. Water is then allowed to flow in and to 
fill the chamber again, which drives out the remainder of the 
former charge of air, and this chamberful of water is driven out 
and up by another charge of full-pressure air as before, and so on. 

With the return-air system, however, the air is not let off so 
easily, but is retained to do a lot more work. The air is dis- 
charged from the water chamber and up the air pipe which, by 
the action of an automatic or a positively operated switch, is 
now in connection with the intake instead of the discharge of the 
compressor, and whatever pressure this air may have is thus 
always being exerted upon the intake side of the compressing 
piston, and the pressure upon this side overcomes an equal amount 



COMPRESSED AIR FOR RAISING WATER 249 

of resistance upon the compressing side of the piston, or it deducts 
just so much from the total power required to drive the piston 
and to do the work of re-compression. 

Using the air in this way, its entire expansive force is retained 
upon the credit side of the system. Expansion continues until 
the air-pressure is lower than that of the water which is waiting to 
enter, the difference of these pressures depending upon the speed 
at which the system is being driven, and then the water enters 
and drives the rem.ainder of the air into the compressor intake at 
this pressure, instead of letting it further expand, and thus raises 
the total mean effective assisting pressure at the back of the pis- 
ton above that which would have resulted from the expansion 
alone. 

Each return-pipe installation is an individuality, with special 
conditions to be satisfied and calling for special sizes and capaci- 
ties, both relative and actual. The system is advantageously 
applicable to both extremes of the possible range of practical 
requirements, and to all between. 

Constant Lift System. — Where the lift is practically constant, 
as in raising water from a lake or river to a water-works reservoir, 
the compressor and the pump may be so adapted in capacity as 
to utilize all the expansive force of the air with comparatively 
little of the follow-up pressure from the water, but still with the 
highest possible ultimate efficiency and economy. In any case, 
as the compressor is in immediate touch with its work there is 
no loss from overcompression or from compressing above the 
actual requirement, as is often the case with the direct-displace- 
ment systems. 

Variable Lift System. — If the return-air system is employed 
where the level of the water intake has a considerable variation 
of height, as in emptying a large, deep shaft of a mine, the follow- 
up pressure of the water in refilling the operating chambers will 
vary constantly with the water-level, and at all levels will approxi- 
mately adjust the work of the compressor according to the actual 
water lift at the time; and, although the water must always be 
lifted under its full actual head at the time, and with the requi- 
site overbalancing pressure for the expelling air, the compressing 
of the air to that pressure will require little or much power accord- 
ing to whether the return, or intake, pressure at the compressor 
is much or little, corresponding to the height of the water-supply. 
With the direct displacement, and no return of thc^ :iir. there 



250 COMPRESSED AIR PRACTICE 

would be none of this compensation, either from the re- expansion 
of the air or from the follow-up pressure of the incoming water, and 
all the water lifted, whether the level of the supply was high or 
low, would require the maximum and entirely uncompensated 
pressure to raise it. 

An Example of Application. — There was a case where it was 
necessary to provide, and to have constantly ready the means of 
emptying a shaft of considerable area, more than 300 ft. deep and 
normally full of water, with a tunnel at the bottom of the shaft 
also to be emptied. This will be recognized at once as a problem 




Fig. 75. — Return Air iDstallation at Harlem River Siphon. 

easy enough of solution, in a way, but not so simple when speed 
and economy were to be considered. There had been actually 
installed for this work a hoisting engine and a large cylindrical 
bucket, say 4 ft. in diameter and 12 ft. long, to be successively 
filled, hoisted and emptied. 

This might strike one as a rather primitive arrangement, but 
it had at least the merit of using the power with reasonable 
economy. The varying height did not at all affect the weight of 
the bucket of water, and the power consumed was therefore 
always directly as the height, and with engines showing good 
steam economy the arrangement was not wasteful of power. 



COMPRESSED AIR FOR RAISING WATER 251 

This bucket arrangement was actually used for emptying the 
shaft and tunnel once and it worked all right, but nevertheless 
it was superseded by the return-air system, which could show 
good power economy and could do the required work in much 
less time, which in this case was the all-important consideration. 

The shaft here spoken of is known as shaft 25 of the ''new" 
Croton (not the Catskill) aqueduct, and the tunnel is the portion 
of the aqueduct which crosses under the Harlem River, 300 ft. 
below its surface, near Washington Bridge, New York City. 
Fig. 75 shows the return-air pump house over shaft 25. 

Valve Arrangement and Operation. — In the foregoing outline 
of the operation of the return-air system reference has been made 
only to the action in one submerged operating chamber. It is 
understood that there are two similar chambers operated alter- 
nately, one filling while the other is emptying, so that the lifting 
is practically constant. The four-way valve or switch, as it is 
called, for reversing the connections of the two pipes at the proper 
times is operated in different ways according to the work. For 
a straight, steady water-works job the switch may be operated 
mechanically, tripping one way or the other for each given number 
of revolutions of the compressor; it may be operated automatic- 
ally by the fluctuations in the level of the water supply, and it 
may also always be operated by hand. It is not necessary to 
completely fill either chamber before reversing, because the un- 
used pressure is not lost in any case, and there is nothing corre- 
sponding to the clearance losses in the direct-pressure ejection 
system. 

The actual installations of the return-air pumping system 
already show a wide and interesting range of successful employ- 
ment; what the device needs most of all is to be better known. 
It seems to decline no job which is possible by any system, it 
works for the lowest air output, and is especially successful in 
many lines which are not permissible to the formal mechanical 
devices. It pumps, for instance, water which is so charged with 
solid matter that it can only be called semi-liquid. It is in some 
cases regularly employed for conveying the solids so contained, 
as sand or marl, the mixture when so raised going to settling 
tanks where the water is allowed to flow off, leaving the solid 
portion to be shoveled or otherwise handled and conveyed wher- 
ever required. 



CHAPTER XXIV 
THE AIR LIFT 

One of the most important, and now also among the most 
extensive and most rapidly extending employments of compressed 
air is in the so-called '^air lift," used for the raising of water and 
other liquids. This name is not correctly suggestive of the 
characteristics of the device, but it seems to be quite firmly fixed 
and no one seems to be able to supplant it with anything better. 

The water-works of many towns and cities in the United States 
and elsewhere are entirely dependent upon the air lift, and its 
use for the obtaining of water from its natural subterranean 
sources is rapidly extending throughout the civilized world. It 
is used largely for the pumping of oil wells, for the unwatering of 
mines, and it handles semi-hquids, or hquids carrying a heavy 
burden of comminuted solid matter, with the same facihty as 
clear water, making the water thus in many cases the cheapest 
conveyor available. 

The adoption of the air Hft for any specific service is seldom 
determined upon by considerations of power economy alone. It 
has its special adaptations to some exacting conditions, and it 
may be said to hold its own and to be continually extending its 
holdings by the fact that it can do work to which no other devices 
can be applied. The wide range of its employments, and its 
continuance in them is the best possible evidence that after all it 
pays. It is also free for anyone to employ it, as there can be no 
monopoly of the principle involved. 

It is not necessary to make any mystery of the operation of the 
air lift, and no one has any privilege to pose as an exclusive reposi- 
tory of special information concerning it, although in the practical 
applications of it experience counts for more, and is more abso- 
lutely necessary than in almost any other field. 

Principle of Operation. — The essential principle involved seems 
to be very simple and to be easy of explanation. Say that we 
have an open well, or in fact any body of water, so that it be deep 
enough, and regardless of its volume or what may enclose it, a 

252 







• 


'IT 


'■ ^^:>c" 


:-^'^-i^-..i^i 














m^^:^ 








■ • w 








♦". . 


-^"^^^■■■■■H 


5 W ^- >:■' 






•.■-■■ • ..'•'»■ .V- , 


v,.V->- - 


/'•^ ^^^1 




0^- ■ 




S' 


^Oik^i^H 


l'^'# 


^ 


fj;^^^:*?^''^ . ■ ,^*ip^g«v 


■^ife'- 


-/^^H^sPSw; 


2rV: 


i|''"'' ^■'•■^^--'■•■:' 


h^^^ 






'^^I^^^^^^^^^^^M 


*:'' ' '■ '.?! '.'■ ■■■ 






HFT'- 






-^ 


P „ ,,.., 




1 


J 


i^^;=^, 


•±J:i: > 


d 


^^^^IpQ^eV 'SmhH 


«> 


^b^-'^^'^ ltd 







<i 



41 



THE AIR LIFT 



253 



^^^Px 



^ 



From Com. 
pressor 



pond or lake or river being as good for our purpose as a well of 
whatever dimensions; and say that in this water is suspended or 
fixed a vertical pipe with open ends, the lower end extending a 
considerable distance below the surface of the water, the water 
being thus perfectly free to enter the pipe. Under the conditions, 
the water will rise to the same height inside the pipe as it is out- 
side, and when the water-levels coincide there will be nothing 
operative to cause any movement of the 
water either upward or downward. 

If we wish to induce the water inside 
the pipe to rise higher than the water 
outside the pipe it can be done by re- 
ducing the weight, or more correctly, the 
specific gravity of the column of water 
as a whole contained within the pipe, and 
then the greater relative weight of the 
water outside the pipe will act to force 
the lighter column of water upward. 
The simplest way in the world to lighten 
the column of water in the pipe would 
seem to be to mix air with it, and this 
the so-called ''air lift" does, and it is 
practically all that it does, while gravity 
does the rest. 

Say that, with our pipe standing in the 
water as above assumed, and "with the 
water standing at the same height within 
it as outside, a compressed-air pipe con- 
siderably smaller than the other is run 

down alongside with its lower end turned up and reaching a 
little distance up into the large pipe, as in Fig. 77. In the plac- 
ing of this air-pipe it may be supposed that the water has entered 
it and has risen in it to the normal water-level, if there has been 
no confined or compressed air in the pipe to oppose it. If now 
compressed air is turned on to the small pipe with a pressure suffi- 
cient to drive the water down to the lowest point in the bend of 
the pipe, the pressure required being determined by the height 
of the water surrounding the pipe and this pressure, 1 lb. for 
each 2 ft. (speaking loosely) of water depth, being the greatest 
pressure that will be called for in the operating of this particular 
lift, then as soon as the bend in the pipe is passed the air will 



c^ 



Fig. 



77.— The Principle 
of the Air Lift. 



254 COMPRESSED AIR PRACTICE 

escape upward out of the air- pipe and will diffuse itself through 
the column of water above. 

We must not go too fast here. The air " hft " has not yet begun 
its lifting. The diffusion of the air through the column of water 
elongates the column of water upward, but does not yet lighten 
it as a whole. The air might go on mixing with and diluting the 
water and gradually working its way up through it and finally, if the 
pipe was long enough, the air would escape at the top of the col- 
umn as fast as it was admitted at the bottom, but still there 
would be no ''lifting" of the water by this operation alone. This 
gradual working of its way up through the water which the air 
thus does in the stationary column goes on also, but not so rapidly, 
when the column is moving and when the lift is in full operation, 
this constituting the ''slippage" which seems to be one of the un- 
avoidable sources of loss in the air lift. 

As a matter of fact in the air lift the air does not work its way 
up through a column of water of unlimited length and escape 
out of the water at the top, leaving the water inert. Before this 
height is reached the water pipe ends and there is a continuous 
discharge of its contents. This discharge of the upper portion 
of mingled water and air at once reduces the weight of the column 
and then it begins to be pushed upward by the heavier solid 
water which is now able to force its way in at the lower end. The 
escape or discharge at the top makes the column continually 
lighter than is required to balance and entirely resist the in-press- 
ing water below, so that the solid water continues to flow in, and 
then the entire column is kept rising and flowing off at the top, 
as long as the supply of compressed air is sufficient to maintain 
this condition of unbalanced pressure; and there we have the air 
lift complete. It is ultimatelj^ worked as we see entirely by the 
force of gravity. 

It will be better to finish with the air and its career in the water 
before considering the action of the resulting unbalanced pres- 
sures. The levitation function of the air begin understood, it 
will be evident that it must be desirable to have it mixed as 
intimately as possible with all the water in the column, so that 
here ingenuity and invention may play an important part in 
providing means for scattering the air and mixing it at the begin- 
ing all through all the water. If such scattering and intimate 
mixing of the air with the water does not occur at the instant 
of its discharge from the air-pipe no such result can be possible 



THE AIR LIFT 255 

later. It is to be assumed that the air will be distributed through 
the water in bubbles, large or small, and it is better that they 
should be as small and as numerous and as nearly as possible at 
uniform distances apart rather than to have them fewer and 
larger and irregularly distributed. 

The originally propounded theory of layers of air alternating 
with pistons of water in the pipe represents an absurd impossi- 
bility. Whatever the number and size of the air bubbles at the 
beginning they will always be merging together to form fewer 
and larger ones. Whether the aerated column be stationary, as 
first assumed, or moving upward with the lift in full operation, 
the levity of the column which the air is employed to produce 
will be determined by the amount of air actually distributed 
through it during the period of its upward passage through the 
pipe. If some of the air escapes by working up through the 
column, instead of moving with it, it represents power lost, and 
this loss will be greater with large bubbles than with small, 
because they will always float up through the water at the greater 
speed. 

Air Used Isothermally. — In its operation the air lift uses the 
air expansively and gets the benefit of the expansion. When the 
lift is in full operation and the air and water mixture is flowing 
at a uniform speed through the discharge pipe there will be at any 
instant a given quantity of water and the equivalent of a given 
volume of free air. The air actually will not be either free air 
nor air at the full working pressure. When it is first discharged 
from the air pipe and mingles with the water it has already lost 
a small portion of its pressure, and as it rises with the column it 
will expand as the pressure due to the depth decreases, and it 
will be discharged with the water at a pressure little above that 
of free air. As the value of the air in the column is in the quan- 
tity of water it displaces, this displacement will be greater per 
volume of free air at the top of the column than it will be any- 
where below it. As the air is so intimately mixed with the water 
it must be assumed to take the temperature of the water, or its 
expansion may be said to be practically isothermal, which is as 
profitable to the user of the air when expanding as the absence 
of this condition is unprofitable when the air is being compressed. 

Compressed air in the air lift, it has been seen, does not push 
or force the column of water upward after the lightening of the 
column, the latter being its only function, ;ind the flow upward 



256 COMPRESSED AIR PRACTICE 

is caused by the unbalanced weight of the sohd water outside and 
below the pipe. This suggests at once the necessity of main- 
taining a sufficient preponderance of pressure from theinrushing 
water, and this is only secured by the submergence of the pipe or 
its extension to a sufficient depth below the surface of the water. 
The height of the lift is the principal factor in determining the 
submergence required, the invariable condition being that the 
higher the lift the greater shall be the actual submergence. The 
percentage of submergence, spoken of later, is another matter. 

In practice the matter of submergence assumes the greatest 
importance, and mistakes or misjudgments in this particular 
are not infrequent. In planning an air lift for a bored and tubed 
well wherein the water does not rise to the surface but stands 
constantly at a certain distance below the surface, it is com- 
paratively easy to compute approximately the submergence 
which should be adopted for delivering the water at a given 
height, experience rather than theory dictating the figures; but 
if when the pumping begins the height of the water in the well 
falls any considerable distance, as it often does, then the propor- 
tionate and the actual submergence may be quite inadequate. 
The submergence will be reduced as many feet as the surface of 
the water supply falls, and the lift will be increased an equal 
distance, a double-acting cause of incompetence. In manj^ cases 
where a new air lift is installed provision is made for lengthening 
or shortening the pipes to get the best submergence according 
to the developed working conditions which could not have been 
anticipated. 

With a given submergence the lift may be greater or less, 
within wide limits, and the lift will still work, but there is for 
each case an approximate ratio of submergence to lift which gives 
the best results, both as to the volume of liquid raised and also 
as to the air cost, or power cost, of raising it. With a fixed sub- 
mergence, whether the lift be proportionately small or whether 
it be as high as possible, the air-pressure required will be the same, 
but while the pressure is thus determined by the submergence the 
actual volume of air per unit of time, and also the volume of 
water raised will be influenced and determined by other condi- 
tions, especially by the actual volume of air supplied and the 
resultant rate of flow of the water. 

Details of the Air Lift. — In computations upon the air lift the 
submergences are stated in percentages. The submergence 



THE AIR LIFT 



257 



percentage is that portion of the whole length of the discharge 
pipe which is submerged, the remaining percentage constituting 
the lift. Thus with a lift of 100 ft., if the submergence is 60 
per cent., the lift, or 100 ft., will be 40 per cent. The 60 per cent., 
the submergence, will therefore be 150 ft., and the total vertical 
length of the water or discharge pipe will be 100+150 = 250 ft. 







1 


I 


% 


I 


Seal 


e for 

1 


Constant C 


oo 


2 


8 


3 




















































































/ 




Is 




































/ 


/ 






-200- 


















Con 


itant 


C . 










■ 




7 


/ 








— ■ 






— ■ 




















/ 


/ 


y 


1 






























_/ 


y 


/ 


'y 


/ 


V 






























^^ 


y 


,/ 




/ 


/y 


< 






V = 


L 














y 


X 


y 


y 




/ 


/ 




r„_ S+34 ^,„ 


t ' 
















^ 


y 


y 




y 


y 


y 


/ 












VCf^ 


.e^ 


c^ 


^ 








u' 


/ 


y 


















.Z^J. 


L^ 


^ 


^ 






^ 






y^ 


y 








9 












s 


\^s^ 


^ 








^ 




JO^ 


^ 


^ 
















^ 


^ 


^* 




^ 


.^ 




s 


^ 


^ 




'(^ 














1 


^ 




^ 




^ 


^ 




-.^ 




r^ 






















^ 


^ 


-i:^^ 


^ 


--^ 


^^ri 
































^ 


^:^ 












1 



























§ 8 'S 



i 



§ 



Height of Llf t-Ft. 

Fig. 78. — Free Air Requirements for Different Submergences. 

In Fig. 77 il5 is the lift and BC is the submergence, both being 
stated in percentages of the total length AC. 

Attention is now directed to the chart or diagram. Fig. 78 
from which may be readily determined the volume of free air 
required with different percentages of submergence to raise 1 gal. 
of water to different heights from 20 ft. to 1000 ft. The data 
embodied in this diagram are practically the same, except in one 
particular, as the figures of a table given to the public a few years 



258 COMPRESSED AIR PRACTICE 

ago by Mr. E. A. Rix, of San Francisco, who has done much in 
various ways for general compressed air practice, the table hav- 
ing been compiled by Mr. Geo. H. Reichard of the same city. 

The formula by which the table spoken of was computed is as 
follows : 

T. 

F = - 



Log.^^ X C 



Here V is the volume of free air in cubic feet, this being taken 
in this case as the actual piston displacement of the compressor. 

L is the lift in feet. 

C is a constant, 234, unchanged throughout the computing 
of the table. 

S is the actual submergence in feet, which is obtained as follows: 

LXsp 



S = 



Ip 



sp being the submergence percentage and Ip the lift percentage. 
Say that the lift is 100 ft. and the submergence 60 per cent., 
then: 

100><60_ 
'^"100-60"-^^^ 

Using figures now in the original formula we have: 

Log. -^X234 

For the air-pressure required for any lift, and with any per- 
centage of submergence, it is convenient to divide the actual 
submergence in feet by 2 to get the gage pressure in pounds. 
This gives enough pressure in excess of that due to the water 
head to allow for the pipe-friction and other losses. 

The one particular in which the computations for our diagram 
differed from those for the Rix table spoken of was in the constant 
C. This, as indicated by the line in the upper part of the dia- 
gram, instead of remaining the same, 234, all through, was made 
to decrease gradually as the lift increased. Thus for a lift of 
200 ft., our constant is 235, practically the same as that used by 
Mr. Reichard, but for 100-ft. hft it is 245, for 500-ft. it is 200, and 



THE AIR LIFT 259 

so on. The result of this change is to increase more rapidly the 
free air per gallon as the lift increases, which seems to agree 
better with observed results. 

The Une for constant C in the diagram follows very closely 
the figures given by Mr. H. T. Abrams in a lecture at Columbia 
University. 

The most striking thing observable in an examination of this 
diagram is that the volume of free air required per gallon of water 
lift constantly decreases as the submergence increases. Thus at 
25 per cent, submergence and 200-ft. Hft, the free air required 
per gallon is 1.83 cu. ft., while for the same lift and 60 per cent, 
submergence the free air is 0.88 cu. ft., or less than one-half. In 
the first case, however, the air pressure required is about 32 lb. 
gage, while in the other case the pressure would be 140 lb., and 
the mean effective pressure required for the compression of the 
air (adiabatic) to 32 lb., would be about 20 lb., while for compress- 
ing to 140 lb. the mean effective would be 50 lb. Then compar- 
ing 1 .83 X 20 = 36.6 with 0.88 X 50 = 44, it appears at once that the 
apparent cheapness of deep submergence is nullified or reversed, 
and other considerations determine what submergence is best. 

While, as was said farther back, the higher the lift the greater 
must be the actual submergence, the percentage of submergence 
will be the other way. It has been recommended in some 
quarters that a submergence of 60 per cent, be adopted in all cases, 
but better results are obtained by departing from this in both 
directions, going to 65 per cent, for lifts of 20 ft. or so and down 
to 40 per cent, for lifts of 500 ft. or more to secure the greatest 
delivery. This is in recognition of the augmented pipe friction 
when the vertical length of pipe to be traversed is so great. 

The air lift is intended for steady work and not for occasional 
and frequently intermitting service. It is not to be stopped and 
started at any odd time like a common reciprocating lifting pump. 
Its efficiency is largely influenced by the correct adaptation, or 
otherwise, of the sizes of air-pipe and discharge pipe to each other 
and to the rate of air delivery, the latter being entirely controlla- 
ble, while the inflow of the water adjusts itself to the conditions 
provided. Nothing is thought of here but the relations of dis- 
charge pipe and air-pipe to each other, it being assumed tempo- 
rarily that the water is free to flow into the bottom of the discharge 
pipe as the ''air Hft" takes it away. The speed of this voluntary 
(as we may call it) and uncoercible flow of the water really deter- 

19 



260 COMPRESSED AIR PRACTICE 

mines the best working speed, and also, within certain limits, the 
possible working speed of the air lift. As might naturally be 
supposed, the rates of flow in the pipes and the corresponding 
actual deliveries of water will be more rapid for the low lifts and 
small actual submergence than for the high lifts and deep sub- 
mergence, the greater length of pipe causing the greater retarding 
friction. 

While from what has been written above it would seem that 
almost any one might install and operate an air hft, there is prob- 
ably no application of compressed air in which experience counts 
for so much, and it is recommended that in all cases a competent 
expert be consulted or employed. In this view of the case it 
odes not seem worth while to lengthen this article by the insertion 
of tables of pipe capacities and other working data, as these may 
be obtained from builders of air- compressors and others in touch 
with the business. 

The writer is tempted, however, at this point to offer a sugges- 
tion as to a novel application of the air lift which seems to be 
altogether feasible and which it may be worth while to think and 
talk about. 



FOR A CATSKILL AQUEDUCT MONUMENT 

Probably few persons have any adequate and abiding apprecia- 
tion of the fact that the great engineering works of the day are so 
largely indebted to compressed air for their existence. This is 
especially true of the great aqueduct which is to bring the water 
of the Catskills and distribute it over the entire area of Greater 
New York. It is not now possible to see how without the 
sustained activity of the air-driven rock drill the aqueduct could 
ever have been. 

That this is the simple fact is indisputable, and it would seem 
to furnish us sufficient warrant to speak of the great aqueduct 
in Compressed Air Practice, especially as we here purpose to lay 
it under additional obligation to pneumatic agencies. 

It is seriously proposed, and the project seems to be under way, 
to erect an imposing monument to the Panama Canal. The 
gratuitous inadequacy of the proposition resides in the fact that 
the great work is in itself all monument. It is all open to the 
view, and both in detail and in its entirety it tells its own story, 
impressing the beholder with its magnitude in a way to belittle 



THE AIR LIFT 261 

and render futile any artistic or architectural commemoration 
that could be devised. 

The same will be true of the New York State Barge Canal 
when completed, of the tunnels and subways of New York City, 
and of the city itself, when it is finished. Rome required no 
monument besides itself to perpetuate its renown. 

There are, however, other great and costly works for the service 
of the people which are in no respect spectacular, or even visible 
at all, and which have no opportunity to appeal to the eye with 
any impressive reminder. This is especially true of our modern 
pressure water tunnels which give to the world no sign of their 
existence, and which it is easier to forget than to appreciatively 
remember. The aqueducts of the ancients were eminently 
monumental, and when they survive they suggest adequate con- 
ceptions both of the works themselves and of the communities 
which built them. 

The original Croton aqueduct, which followed the ancient 
practice in providing a level flow for the water, has a worthy and 
enduring monument for itself in its own ''High Bridge" which 
carries it across the Harlem River. The "New" Croton aque- 
duct, which is a much greater work, but which, being a pressure 
tunnel, is all underground, has nothing to show but a gate house. 

The Catskill aqueduct, of whose magnitude and magnificence 
as an engineering work it is idle to speak, also functionally pro- 
vides no monument for itself; but what material achievement of 
man could more loudly call for a worthy visible and enduring 
reminder to compel the appreciation of its myriads of beneficiaries 
who may be mostly yet unborn? 

The only appropriate type of monument compellingly suggests 
itself at once. Nothing can be thought of but a fountain. 

It happens that a fountain regardless of its specific commemora- 
tive function in each case, is the most effective and satisfactory 
of all monumental devices. No better work has been done by 
the artist-architects of the world than in the designing and erect- 
ing of fountains, and these works are the most conspicuous and 
the best remembered of the sights which are the boast of the great 
cities of the world. 

Too often it has been that in the fountains which should 
have been most admired and renowned there has been too much 
art and too little water. Magnificent erections of stone have been 
provided with imposing statuary in marble or in bronze, and one 



262 



COMPRESSED AIR PRACTICE 



or more little trickling streams of water, and many times the 
sight- seeing traveler has to turn away in disgust when he finds 
these fountains dry. 

The cost of the erection of the fountain ends with the comple- 
tion of the work, but that of the water to animate it continues. 
Water costs for its storage and conveyance, and frequently also 
there are constant pumping charges, and after the water has 
performed its function in the fountain it all runs to waste, so that 
the cost of the fountain may be said to begin rather than to end 
with the first fiow of the water. 

Now, in the general water supply scheme of Greater New York, 
as backed by the Catskill aqueduct, there is a unique opportunity 
as regards the providing and maintaining of a spectacular com- 
memorative fountain. Here may be a fountain which will be 
a novelty to begin with, in that it will be chiefly if not entirely a 
fountain of water, and not a fountain of stone or of bronze with 
a Httle water to save the name of it. The water for the fountain 
may be in practically unHmited quantity, it will cost nothing 
for its flow at the fountain, and none of it, except the Httle that 
evaporates, will be diverted from the ultimate channels of con- 
sumption. All the art called for from the designers of such a 
fountain would be for the spectacular arrangement of the water 
flow, but in that might be found an opportunity such as never was 
before. 

All this is so simply because the flow of the aqueduct is to 
come into the heart of the city with a pressure much above that 
which can be used for the lower or general service. When the 
water enters the distributing reservoir — say the present large 
reservoir in Central Park — let it, all of it or as much of it as may 
be required, enter by an overflow at such a height as the full 
pressure will give it, instead of by a subterranean and restricted 
channel; and there we have our fountain. Not a word will be 
added here as to the details of it, if only an unassisted gravity 
fountain is to be considered, and even that should make a very 
satisfying exhibit. 

But so much better than this could be done. The possibilities 
of the air lift for fountain purposes, it is believed, have never been 
exploited. The water discharged by the air hft is not, as we 
might say, '' solid " water, such as would be dehvered by a mechan- 
ically operated pump, but is water mixed with much more than 
its own volume of air, and consequently the discharge for any 



THE AIR LIFT 263 

given quantity of water per unit of time with the air left is at a 
much higher velocity than it would be with the water unaerated. 
Fig. 79 is a noble stream of fully aerated water from a 6-in. 
air-lift pipe. The same quantity of water coming from a steam 
pump of the same capacity would not show nearly the life or the 
bulk of this stream. The pump stream would show the color of 
the water, and if not too oily it would be more or less transparent 




Fig. 79. — A Typical Air Lift Discharge. 

This stream, as we see, is beautifully white with no suggestion 
of transparency. Not only is the visible volume and velocity 
increased by the air lift, but the jet has a Hght and feathery 
effect which cannot be produced by any other means. Fig. 80 
gives some idea of the fairy lightness of the air-hft stream, al- 
though the photo was not taken for the purpose of showing this, 
and better effects may be produced where a suitable background 
is provided for the display. This suggests the necessity of prop- 



264 



COMPRESSED AIR PRACTICE 



erly locating the fountain we are suggesting and the position 
for it might not be in the middle of a large body of water, which 
is the situation which has frequently been chosen. 




Fig. 80.— FountaiD Effect of Air Lift. 



But it is idle to speak of the display which might be made with 
air-lift jets, or with such jets in connection with gravity discharges, 
unless we have a sufficient supply of compressed air for the lifts. 
This also the water of the aqueduct could easily be made to pro- 



THE AIR LIFT 265 

vide in its necessary transmissions from the higher to lower 
pressures, with no cost except for the construction of the com- 
pressing plant; and this might be located wherever most con- 
venient. As appeared in our chapter on the Taylor air- compres- 
sor, any fall of water, whatever its height, can be made to compress 
air to any pressure desired, the only imperative condition being 
that the volume of falling water shall be sufficient for the work. 
The Catskill aqueduct seems to be well provided with both the 
water and the power for a monumental fountain worthy of itself 
and of the Metropolis. 



CHAPTER XXV 
AIR FOR LARGE STEAM HAMMERS 

The use of compressed air for driving large hammers such as 
are normally steam driven, in boiler making, ship building, bridge 
work, and general heavy forging is rapidly extending. Not only 
are many original steam hammers now so operated, but they are 
always operated with satisfaction if not always with computable 
and demonstrable power economy, and when once air is employed 
for the purpose its use is rarely if ever abandoned. The con- 
siderations urging this use of air are quite convincing. 

It has come in my way to know something of the manufacture 
of rock drills. These drills are built to be operated some by 
compressed air and some by steam. A steam drill, by the way, 
may always be operated by steam, but an air drill cannot always 
be operated by steam on account of its destructive effect upon 
leather packing. Every drill is tested or, as we might say, 
broken in, at the factory before it is sent out. This testing 
operation is much more thorough and exacting than the term 
would suggest. Each drill is run for a considerable time under 
different conditions of stroke, pressure, speed, etc., until it is 
found to work correctly under all normal conditions. 

It happens that all steam drills while thus being tested are 
run first of all with air and after that with steam, and after the 
drill has been made to work all right with air it still requires time 
and nore or less coaxing before it works equally well with steam, 
the latter being always the more difficult proposition. The rock 
drill is only a smaller steam hammer, and the testing experience 
with the drill is closely typical of general experience with the 
steam hammer. 

It will readily be conceded that the use of steam in the steam 
hammer is never without a number of objectionable accompani- 
ments. I speak now of the hammer as installed under not un- 
favorable conditions; it may be located not far from the boiler 
and freely suppHed with comparatively dry, live steam. The pis- 
ton or tup is a soUd mass of metal, its weight being largely depended 

266 



AIR FOR LARGE STEAM HAMMERS 267 

on to give force to the blow, and the cylinder also is much heavier 
than that of a stationary engine of the same diameter, and when 
the steam is turned on its first work is to heat up all this mass of 
metal, thus involving not only the condensation of a quantity 
of steam, but also the flooding of everything with water. No 
matter how perfect may be the arrangements for taking care of the 
water, it still works out of the stuffing boxes, drops around when 
it is not wanted, and is the famiUar and constant nuisance of 
the steam operated hammer. The hammer also is never operated 
continuously, so that this warming-up and steam condensing 
operation is repeated more or less every time the hammer is 
started up. The cost of steam wasted by condensation is quite 
an appreciable addition to that of the steam employed for the 
working. There are expansion troubles also connected with the 
use of steam, the parts not heating up and expanding equally 
so that the warming-up process every time the hammer is oper- 
ated also requires the playing of the hammer up and down, the 
working of the valves to have them free, etc., and besides the 
steam consumed considerable time is required. This does not 
necessarily imply any delay of the work, as the hammer maj' be 
got ready ready beforehand, but it takes the time of a man who 
might be doing something else. 

Steam thus charges continually for waiting in readiness as well 
as for the actual work it does, while compressed air costs nothing 
except for work actually done, and this it is always and instantly 
ready for. The hammer the first thing in the morning is readier 
to go with full force the instant the air is turned on than it is 
with steam after fifteen minutes of warming and limbering up, and 
the same warming up is required more or less every time the ham- 
mer is operated. 

Another important point is the lubrication of the hammer. 
With air the oil remains on all the working surfaces the same as 
with machinery which is all exposed, while with steam the oil 
disappears almost immediately and lubrication must be almost 
continuous and requires constant watching. This reliabiUty 
and constancy of lubrication in the one case and the precarious 
uncertainty of it in the other is especially brought out in drill 
testing. 

Three or four different samples of lubricating oil were once 
sent to the drill testing department above spoken of for trial in 
the drills, with the purpose of ascertaining the special adapta- 



268 COMPRESSED AIR PRACTICE 

bility of the different oils for use with steam or with air respec- 
tively. Very little satisfaction resulted from the trial. Which- 
ever sample was tried upon either drill, the uniform result was that 
when the steam drill was taken apart and examined all the sur- 
faces assumed to be lubricated were '^as dry as a bone," the oil 
having entirely disappeared, while the similar parts of the air- 
operated drill were all quite oily. Precisely the same results 
are observed in the lubrication of large hammers accordingly as 
they are driven by steam or by air. 

In considering the matter of air for steam hammers it will 
appear all the way through that it is not to be settled by merely 
comparing the costs at the boiler or the power house. The 
constant readiness, the handiness and liveliness of operation, 
the saving of the time of the workers at the hammer, outweigh 
in each individual case many pounds of coal. 

When the hammer is operated at a distance from the source 
of power, the advantage in the use of the air is more pronounced. 
A steam pipe is of course losing heat and condensing steam all the 
way along, and the steam is wet and heavy when it gets to the 
hammer, while there is practically no loss in the transmission of 
the air, and absolutely no difference in its working readiness at 
the hammer. 

There was recently a specific case up for consideration in which 
if steam was used it would be necessary to pipe it 1800 ft. The 
loss by steam transmission even with costly heat insulation would 
be quite large for this distance. In the case of the air there might 
be a fall of pressure of 1 or 2 lb. this loss being easily computable 
when the conditions are specified, and the loss of pressure 
would be almost entirely compensated for by the corresponding 
increase of volume delivered. The loss by leakage, assuming 
the piping to be properly laid, would be so small as to be entirely 
neghgible. 

We give here what data we have immediately available as to 
the air required for operating a steam hammer. We have infor- 
mation of many plants where steam hammers are driven by com- 
pressed air, but in every case some of the air is used for other 
purposes, so that it is not possible to get the actual air consump- 
tion in any given case, and this would be difficult of ascertain- 
ment for purposes of comparison in any case, on account of the 
intermittent use of the hammers and the difference in the total 
time of employment in each case as compared with any other. 



AIR FOR LARGE STEAM HAMMERS 269 

As a starting item we may note the statement which I have 
from responsible and experienced hammer builders that the 
largest amount of free air required for the continuous running 
of a steam hammer is 26 cu. ft. per minute compressed to 90 lb. 
for each nominal 100 lb. weight of hammer. For hammers used 
in the ordinary way, or with the average of stoppages, the con- 
sumption may be placed at 13 cu. ft. per minute. 

The cost of compressing to 90 lb., two-stage compression, is 
about 0.163 h.p. per cubic foot free air per minute, or for 26 cu. 
ft., as above, 4.24 h.p. and for 13 cu. ft. 2.12 h.p. per 100 lb. of 
hammer. To operate with steam under conditions similar to 
this last instance the same authority says that 1 h.p. of boiler 
capacity should be allowed for each 100 lb. of hammer, the boiler 
located within a reasonable distance. 

The Star Drilling Company, Akron, Ohio, has three hammers 
with an aggregate weight of 4200 lb. At the lowest j&gure given 
above these would require 42X13 = 546 cu. ft., and the horse- 
power required would be 42X2.12 = 89. These hammers take 
care of six fires, and sometimes eight, and are supplied by two 
compressors whose nominal (greater than actual) free air capacity 
is 336 and 296 cu. ft. respectively, one-quarter of the air being 
used for other purposes. We have then 336-f-296 cu. ft. =632 
— 177 (one-fourth) =455 cu. ft., or 98 cu. ft., less than the lowest 
called for by the above rule. The compressors are driven by 
gas engines using natural gas; the power cost chargeable to the 
hammers is $52.28 per month. 

Of the advantage in the use of air in this plant it is noted as 
follows: 

''The hammers do more work than with steam. There is no 
trouble in starting to work out the water. Hammers work more 
lively and freely. No hot water dripping and no additional heat. 
The air-pressure is constant, while with steam there are serious 
fluctuations and the blows of the hammers are uneven. No 
steam is used in this plant and the installation is considered more 
economical and satisfactory with the air." 

It is not here contended that it is in all cases advisable to use 
air for operating large hammers. The main blacksmith shop at 
the Phillipsburg, N. J., shops of the Ingersoll-Rand Company, 
the location and arrangement of which, for convenience, fficiencye 
and economy of operation, were given careful consideration, is 
very near the main boiler plant of the works, and the score of 



270 COMPRESSED AIR PRACTICE 

large hammers there are all operated by steam. There is in the 
blacksmith shop itself a Stirling water-tube boiler mounted in 
connection with the reverberatory furnaces to utilize the waste 
heat, this boiler being connected by an equalizing pipe with the 
main steam supply pipe of the works. When at times more steam 
is here generated than is being used by the hammers it goes into 
the main supply, while when, for instance, most of the hammers 
are working at once, the flow is the other way. Here as elsewhere 
the difficulty of ascertaining the actual consumption is apparent, 
but the arrangement in every other respect has been satisfactory. 

In the oil tempering and tool dressing shop of the same works, 
located a little farther from the boilers, there is a hammer of 
medium size driven by air. This also is satisfactory, and the 
best arrangement under the conditions. 

The air and steam consumption of steam hammers where any 
records are obtainable vary widely, as might be expected. 
The West Manufacturing Company, Buffalo, N. Y., tested a 
hammer 9 in. by 15 in. by actually running it continuously at 
1^0 blows per minute, the air at 80 lb. gage, and using it at the 
rate of 230 cu. ft. per minute as measured by the piston displace- 
ment of the compressor. As a matter of fact, in actual, every day 
service, the compressor, running at the capacity above recorded, 
supplies this hammer, also a 7-in. by 12-in. hammer, five 
large air hoists, twelve small hoists, and a number of live air jets 
for blowing off scale, etc., and besides that the compressor 
''unloader," which stops the air compression, is in operation a 
considerable portion of the time. 

The Buffalo Pitts Co. have a two-stage compressor with a 
nominal capacity of 350 cu. ft. of free air per minute, maintain- 
ing a pressure of 100 lb., which runs a 700-lb. hammer at 150 blows 
per minute. This compressor in regular shop work runs this 
hammer, also a smaller one, an air riveter and a number of 
smaller pneumatic tools. 

The Shiffier Bridge Company, Homestead, Pa., have a two- 
stage, power-driven compressor with a maximum capacity of 
about 500 cu. ft. of free air per minute which drives two steam 
hammers, the air being used also for a great number of other pur- 
poses, the blacksmith shop with the hammers being a long dis- 
tance away. 

At the Painted Post shops of the Ingersoll-Rand Company 
there is a 500-lb. hammer operated by compressed air, and a 



AIR FOR LARGE STEAM HAMMERS 271 

1000-lb. hammer, no longer in use, was so driven. Mr. F. W. 
Parsons, Superintendent, writes as follows : 

''If the hammers were kept pretty busy doubtless steam would 
be the most economical of fuel, but, as they are often operated 
with long waits between, you get with the steam a good deal of 
condensation and this causes lots of bother when starting up, 
the iron cooling rapidly while waiting. Unless the packings are 
very well kept up water of condensation drips on the anvil and 
forging, which is annoying and sometimes dangerous. Of course 
with air there is none of this trouble, there is no loss when the 
hammer is not in operation, and with an equal pressure more 
work will be done using air than if steam were used. With long 
and exposed pipes, as are often used for steam to hammers, and 
especially where the hammers are not kept busy, it is more eco- 
nomical and certainly otherwise more satisfactory to use air." 

The Elliott Frog & Switch Company, East St. Louis, 111., are 
operating a hammer with compressed air because it is so far from 
the boilers, and they have an air supply for general purposes. 
They find that on account of the condensation of steam, etc., 
they operate the hammer to better advantage by the use of the 
air. 

There are many railroad shops using air for steam hammers, 
but we have no specific information available concerning them. 
We never hear of the air being discarded after once being em- 
ployed for this purpose, except for other reasons than those 
immediately pertinent to the hammer service. 

At Ugine, Belgium, in connection with Girod electric steel 
furnaces, there is a large forge containing nine hammers operated 
by compressed air. The ram of the heaviest of these hammers 
weighs 10,000 lb., while the weights of the others range from 2000 
to 200 lb. 

The intermittent use of hammers makes a large air-receiver 
capacity desirable, and the heating of the air immediately before 
it enters the hammer is always promotive of economy The 
Trimvat Manufacturing Company of Boston have a 2000 lb. 
hammer which is suppUed with air at 100 lb. by an electric-driven, 
two-stage air-compressor of 358 cu. ft. free air capacity, the un- 
loader in this case being in operation about a quarter of the time. 
Large receiver capacity and highly efficient re-heating here ex- 
exempHfy their advantages. 

There are three air-receivers 54 in. in diameter by 12 ft. long, and 



272 COMPRESSED AIR PRACTICE 

the air is heated by twenty-four 1-in pipes, 2 ft. 6 in. long, con- 
nected to headers and placed in the hood over the forge fires. 
The heating in this case is so effective that the exhaust from the 
hammers is hot to the hand and has considerable unused pressure. 
The bills for current for this compressor and isolated hammer 
amount to about $140 per month. 

It happens that this company has also a large steam operated 
plant which by the advantage of aggregation, and ignoring all 
the incidental advantages of air service, shows better economy 
for the steam when power cost alone is considered. There are 
one 5000-lb. hammer, six of 2000 lb. each and two of 1000 lb. 
The large hammer is used so infrequently that it is assumed to 
require no more steam than one of the 2000-lb. hammers, the 
aggregate to be operated, then, being thus equal to eight 2000 lb. 
hammers. These are supplied with steam by a 175-h.p. boiler. 
The coal, labor, etc., are estimated to amount to $35 per horse- 
power year, the total cost for the year then being 175X35 = 
$6125, and 6125-^8 = 765 for each hammer, or 765 ^12 = $64 
per month. 

To use air instead of steam no change is required at the 
hammer, except that the exhaust pipe can be dispensed with. 
Pipe up otherwise as for steam and it is ready at once. 
Every one likes the air; it keeps the room cooler for the men; 
there is no water spattering on the hot forging and threatening 
to scald. Part of the exhaust can be used for blowing the scale 
off, giving cleaner and smoother work. The wear on the hammer 
is invariably in favor of the air, the wet steam, when used, wash- 
ing away the oil, deranging the packing and causing leakage of 
piston, valves and stuffing boxes. With air the surfaces pohsh 
like glass, remain constantly more or less oily with little or no 
wear, and the hammer action is lively and prompt at all times. 

There are several large concerns both in the United States and 
abroad now making steam hammers of the familiar types up to 
the largest sizes many of which are used for compressed-air 
service; these, however, it is not necessary to call attention to 
specifically. Full particulars concerning them may be found 
in the publications of °the builders. 

Compound-air Hammers. — There are, however, several com- 
pound pneumatic hammers which are worth considering. Gen- 
erally speaking there is no excuse for compounding the cylinders 
of an air- operated engine or motor unless the air is passed through 



AIR FOR LARGE STEAM HAMMERS 



273 



an efficient re-heater between the cylinders, just as there is no 
reason for two-stage air compression except for the intercooler 
between the stages. In the case of the compressed-air hammer, 
however, there is full warrant for it, inasmuch as it provides 
for the working of the air expansively instead of exhausting it at 
or near full pressure. 

N. S. K. Air Hammer. — In Figs. 81, 82, 83, we have three ver- 
tical sections of the working parts of a, ' 'N. S. K." compound 



Holding Up 



Striking Ordinary Blow 



SErilfing "Dead Blow 




Fig. 81. 



Fig. 82. Fig. 83. 

Figs. 81-83.— Sections of N. S. K. Hammer. 



air hammer built by Peter Pilkington, Limited, Bamber Bridge, 
England. Seventeen of these hammers, by the way, were turned 
out in one batch for Messrs. Harland & Wolff, Belfast. 

There are two vertical tandem cylinders to the hammer, the 
lower one serving as a guide and being provided with a base by 
which it is attached to the hammer standard, while all the air 
work is really done in the upper cylinder. The differential 
piston provides an annular space A, Fig. 81, to which the air is 



274 COMPRESSED AIR PRACTICE 

admitted at full pressure for the upstroke of the piston. By the 
movement of the hand-lever-operated piston valve, after the 
upstroke is made, communication is opened between the annular 
space A, below the piston and the full area of the top of the piston. 
As the piston descends the air works expansively until at the 
termination of the stroke the air-pressure has fallen to very near 
that of the atmosphere, and all the pressure has been utilized on 
the way down. The movement of the valve for the admission 
of air to the annular space for the next upstroke also opens a 
passage for the exhaust of the air above the piston. 

This cycle of operation is followed for the ordinary working of 
the hammer, and it results in a large saving of air over the use 
and discharge of it without the expansive working. When a 
very heavy blow is to be struck the valve is moved to its extreme 
limit of travel, as in Fig. 83, and then the air is admitted at full 
pressure to the top of the piston, giving great force to the blow. 

The Massey Hammer. — Figs. 84, 85 and 86 show the principle 
and made of action of the Massey hammer, built by B. and S. 
Massey, Openshaw, England. This hammer uses the air expan- 
sively for all blows and minimizes the clearance losses. A further 
saving is in the prevention of the usual ''after-flow" of the air 
into t he upper end of the cylinder following the striking of the 
blow. This afterflow is induced by the piston traveling so fast 
in latter portion of the stroke that the air cannot follow quickly 
enough to maintain the pressure, and then after the blow is 
struck, the inflow of air to equalize the pressure is clearly a waste 
of air. In this hammer, this afterflow is prevented by the ar- 
rangement for working expansively. 

Referring to the cuts, it will be seen that the lower end of the 
cyUnder is always in free communication with the air supply 
through port A , the constant pressure serving to keep the piston 
normally in the raised position as in Fig. 84. The air inside the 
ram below the central guide or plunger is used as a spring or 
cushion. The combined effect of the two pressures — the con- 
stant one below the piston and the varying pressure in the middle 
of it — gives a resultant upward force which decreases as the ram 
approaches the top, with a cushion effect at the last, a rapid and 
hvely action being thus produced. When the piston valve is 
raised, as in Fig. 85, the top of the cylinder is closed to the exhaust 
and put into communication with the bottom of the cylinder 
through the port B, and thus also with the air supply. The 



AIR FOR LARGE STEAM HAMMERS 



275 



compressed air, therefore, passes from the supply on to the top 
side of the piston and drives the ram down. 

As the ram descends the piston passes the port B, and in so 
doing cuts off the supply of air to the top of the cylinder. During 
the remainder of the stroke, therefore, the air already admitted 
is used expansively, and this expansion takes place no matter 
what kind of a blow is struck. For the striking of light blows 
with short stroke a small port can be opened by the attendant's 




Fig. 84. Fig. 85. Fig. 86. 

Figs. 84^83. — Sections of the Massey Hammer. 



lever, admitting just air enough to allow the piston to descend. 
This port is closed again when the valves is moved further for a 
long stroke, while for holding-down purposes a second lever allows 
air at full pressure to flow into the cylinder even when the piston 
is at the bottom of the stroke. The small mushroom valve at G 
is intended for admitting the air-pressure into the space on top 
of the piston in case it has risen above port C. 

The 5 cwt. hammer of this make has a ram_9 1/2 in. in diameter 

20 



276 



COMPRESSED AIR PRACTICE 



with a maximum stroke of 24 in. and weighs complete about 
600 lb. It was operated with air at 45 lb. pressure with the fol- 
lowing data as to power consumption, etc.: 

Maximum: (a) Number of hardest blows per minute, 100; 
(h) cubic feet of air per minute for above, 310; (c) capacity of 
compressor for a single 5 cwt. hammer in cubic feet of free air 
per minute, 280; (d) approximate power required for working 
pressure of 451b.; 35 b.h.p. 




The Musker Hammer. 



Average: (c) Cubic feet of free air per minute, 38; (/) approxi- 
mate power required for pressure of 45 lb., 4.5 b.h.p. These 
powers are based on the assumption that the compressor requires 
for a pressure of 45 lb., 12.5 b.h.p. for 100 cu. ft. of free air per 
minute. 

The Musker Hammer. — This hammer, as will be seen, Fig. 87 
has two cylinders, the lower of which is operated only upon the 
down stroke, while the upper cylinder is, or may be, used for both 



AIR FOR LARGE STEAM HAMMERS 277 

strokes. The operating valve is actuated by the hand lever for 
each stroke as usual for steam hammers. Between this valve 
and the cylinder is what is called a ''miter" valve, the function 
of which will appear. Assuming the tup or ram to be at the top 
and the valve down, as shown, to strike a light blow the valve is 
moved upward only far enough to admit the air above the lower 
piston, the same movement of the valve allowing the air under the 
upper piston to be discharged through the middle port to the 
lower exhaust. 

To raise the ram the valve is lowered sufficiently to allow the 
air supply to enter the middle port, exhausting the air from above 
the lower piston through the lower port to the lower exhaust. 
The miter valve is opened by the exhausting of the air. 

To give a heavy blow the valve is moved upward, allowing 
the air to go through the upper port, under the miter valve, and 
to act upon the upper side of the upper piston and at the same 
time through the center of the valve of the lower part, and to 
act upon the top of the lower piston, the air then acting down- 
ward upon both pistons at the same time. 

For the upstroke the valve is reversed; air-pressure is admitted 
underneath the upper piston through the middle port, and the air 
above both pistons is exhausted through the upper and the lower 
ports respectively. 

The miter valve serves for using the air expansively when strik- 
ing heavy blows. It is arranged to be kept open by a curved 
lever arm resting against the ram until the latter descends about 
half-stroke, when the arm is freed and the valve descends, cutting 
off the air-pressure, the stroke being completed by the expansion 
of the air already in the cylinder. 

Economy of working results from both the using of the lower 
piston only for light blows and from the expansive use of the air 
when the upper piston with the large area is used. A compara- 
tive test of air hammers was made by Sir William Armstrong, 
Whitworth & Co., Limited, with the result, as stated, that the 
Musker hammers used less than half the air required for others. 



CHAPTER XXVI 
DIVING BELL AND CAISSON 

An inverted glass tumbler lowered into a pail of water, the air 
in the tumbler restraining the water from rising within and filling 
it, is typical of one of the most important lines of devices in which 
compressed air is employed as the responsible agent. The prin- 
ciple of water exclusion by air-pressure is so widely applied that 
it is not easy to determine which application of it should come 
first to be spoken of. 

It has for instance, been frequently employed for theatrical 
effects, notably at the Hippodrome, New York City, where 
a large water tank is a prominent and permanent feature of the 
stage paraphernalia. Perhaps one of the most striking of the 
effects produced, and one most easily explained, was where a 
considerable body of men, and also of women, went four abreast 
in steady march time down a stairway into the water until their 
heads were submerged and they disappeared entirely not to be 
seen again. 

The sketch here given, Fig. 89, is purely imaginary and is made 
by the writer without any knowledge of the actual details. A 
is the surface of the water in the tank and 5 is a fixed box without 
bottom constituting the exact counterpart of the inverted tum- 
bler. The water does not rise within this box because it is kept 
filled with air at a pressure of not more than 2 or 3 lb. to the square 
inch, sufficient to keep the water out. Immediately before the 
amphibian marching act the surface of the water in the tank was 
agitated and large and numerous air bubbles began rising, these 
showing that the chamber B was full of air which had expelled 
the water and that some excess of air was escaping under the lower 
edge. When the march was on, and the first row had just 
reached the lower step and their heads had just disappeared below 
the surface of the water, they had only to duck their beads to 
pass under the edge of the chamber and theji when they stood 
upright with their heads within the chamber they would at once 
begin to breathe again. Then the chamber being continued as a 

278 




T 



DIVING BELL AND CAISSON 279 

narrow passage to the side of the stage they could turn at once 
and march single file with their heads out of water and be out 
of the way of the row of marchers following them. The passage 
extending beyond the visible side of the stage then steps could 
lead the marchers up and out of the water, and an air lock would 
release them from the air-pressure. The pressure being, as was 
said, not more than 3 lb., the marchers would not experience seri- 
ous inconvenience on that account. The compressor employed 
would not require to be anything more than a foundry pressure 
blower, and the air lock might be simply a revolving door, such 
as we are familiar with in office buildings. 

The diving bell which is the inverted tumbler upon a large 
scale, and like it movable in any direction, is now seldom used or 

Stage Level 



Fig. 89. — For the Submarine March. 

even thought of, and in its day it did very little notable service. 
It could be lowered over anything not too deeply submerged and 
the occupant free to breathe could reach down into the water 
and do things, but the limitations of work in such conditions 
are evident. 

From the diving bell to the caisson is but a step so far as the 
principle involved, but the latter has become the responsible and 
effective device rehed upon for the execution of the most impor- 
tant engineering works. It has become apparently a necessity 
for the construction of bridge piers and abutments, for light- 
houses, for dams and locks or for any structures whose founda- 
tions must be laid below the water level. The caisson 
so employed differs from the diving bell in that when once put 
into use it is not movable, except vertically downward as the 
work proceeds, and that when the required depth is attained, by 
reaching rock or reliable bearing strata, it remains there to be 



280 COMPRESSED AIR PRACTICE 

filled with solid material and becomes a permanent part of the 
structure to be erected. 

The caisson is now perhaps more frequently employed in what 
in the beginning would seem to be strictly terra firma operations. 
The buildings which modern enterprise finds it profitable to 
erect reach to great heights and are of so great weight that the 
foundations must go to great depths to find unjdelding support. 
In lower New York City especially, but also in many other places, 
it happens that before the required depth is reached the water 
begins to force its way in, and caissons are as much required here 
for the exclusion of the water as in strictly subaqueous opera- 
tions. Practically all the skyscrapers of lower New York City 
have pneumatic caissons for their foundations, and the '^sand 
hogs" who find special employment in the sinking of the caissons 
have becom.e sufficiently numerous to boast of their occupations 
as a distinct trade, with rights and regulations and requirements 
of its own. Thanks to the caisson, the erections of the last 
quarter of a cf n'ury in our modern cities may boast a permanent 
stabihty at the base such as is revealing itself as wofully lacking 
in some of the most celebrated ancient and medieval structures 
of the Old World. 

When the inverted tumbler is first lowered, and when the tip 
of it just touches the surface of the water, the air is at once pre- 
vented from escaping, but the volume and pressure of the air 
are as yet unchanged. As soon as the tumbler is lowered at all 
the water rises more or less within it, there is at once a certain 
increase of pressure in the air and its volume is reduced. The 
pressure to which the air is subjected under these conditions is 
the sum of the normal atmospheric pressure at the time and the 
pressure due to the difference in the height of the water within 
the tumbler and the height of the surface of the water outside. 

In the larger practical tumbler, or caisson, each foot of dif- 
ference in the height of the water outside the caisson and inside 
is equal to 0.4466 lb. per square inch. This is the pressure 
which would be shown upon a gage exposed to the pressure of 
the atmosphere and connected by a tube or otherwise with the 
interior of the caisson. In a caisson more or less submerged 
the abnormal pressure to which the workers in the caisson would 
be exposed would be that due to the water height outside it. 
If this height were 50 ft. then the pressure within the caisson 
would be 50X0.4466 = 22.33 lb., gage. 



DIVING BELL AND CAISSON 281 

In practice the water does not rise in the caisson, the pres- 
sure within being assumed to be sufficient to keep the water 
down. If the pressure of the air were not artificially in- 
creased by the forcing in of more air the water would rise 
to quite a height within the caisson. It would require special 
computation to ascertain the height to which the water would 
thus rise within the caisson, but this never occurs in practice, 
and this is where the work of the air- compressor begins. Suffi- 
cient air-pressure is maintained within the caisson to prevent the 
water from rising within it at all, and this pressure will be that 
due to the outside water height. There is no care required 
at the compressor for the nice adjustment of this pressure; it 
being only necessary to keep the compressor constantly working 
and to send down an excess of air, the surplus escaping under 
the lip of the caisson, so that the pressure required to balance 
the water pressure cannot be exceeded in any case. 

This excess of air supply is a constant necessity, as the air in 
which the men work becomes vitiated by respiration and other- 
wise and would become unbreathable if not renewed. It is 
necessary to take means for keeping the air sent through the com- 
pressor as pure as possible to begin with, and as free as possible 
from oil vapor and other impurities which the compressor might 
contribute, and it is also necessary to deliver the air at or near 
normal temperature, the general effect of the compression being 
to heat the air, so that for caisson and tunnel work an after- 
cooler for the air is extremely desirable, if not imperative. 

The pressure of the air at the bottom of the caisson has a 
Ufting effect upon the entire horizontal area, so that provision 
must be made for weighting the caisson to force it down. As 
the caisson, when sunk to its permanent position at the required 
depth, is to be filled entirely w4th concrete to form a soHd mass 
to sustain the steel columns or other superstructure, as much of 
this permament load as possible is placed upon it during the proc- 
ess of sinking. At the bottom of the caisson is left a chamber 
of sufficient height above the lip for the men to work in, this being 
covered by a deck with a sufficient central opening for men and 
material to pass up and down, this opening or vertical passage 
being extended upward as far as necessary and closed with an air 
lock for retaining the air-pressure. Then upon this deck and 
around the central passage while the sinking is going on and the 
men are working in the pressure chamber, there is built up the solid 



282 COMPRESSED AIR PRACTICE 

mass of concrete or masonry which is to form the permanent 
structure, this being carried up to a considerable height above 
the ground. In addition to this permanent load large weights 
of cast iron are often piled on top to help the sinking. Besides 
the actual air pressure below there is often a large earth friction 
around the sides of the caisson to be overcome. When the lip 
of the caisson has reached the solid rock, or whatever is accepted 
for the permanent bearing, the working chamber below is filled 
up solid and also the working shaft all the way up, the air locks 
and piping being removed. While the filhng up is going on the 
air-pressure is gradually reduced or altogether withdrawn when 
no longer required, and the permanent load rises upon the com- 
pleted foundation. 

The depths to which pneumatic caissons may be sunk is lim- 
ited by the ability of the men to endure the air-pressure, and it 
is a rather curious fact that the depths below the surface at which 
solid rock is found in New York City approach very nearly with- 
out exceeding this limit. In sinking the foundation caissons 
for the city hall the maximum pressure worked in was 45 lbs., 
which is very near the limit record for this kind of work. 

There has been much planning for a bridge across the North 
River at New York, such a bridge requiring one or more piers in 
midstream, but it is considered that pneumatic caissons would be 
impossible on account of the depth and the consequent air-pres- 
sure involved, and so that schem^e waits for some other device 
which would be practicable and not too costly. 

The Caisson for Subaqueous Tunnel Construction. — The two 
halftones (Figs. 88 and 90), help to tell the story of another in- 
teresting type of caisson work. The lines of urban transit in 
Paris cross the Seine five times upon bridges of various design, 
but one line, traversing the central portion of the city, passes 
under the river and in its construction a thin ribbed mode of con- 
struction was employed. The plan first proposed was to have 
two tunnels driven at a suitable depth by the compressed-air 
shield method. This would have necessitated the adoption of 
a level at least 10 ft. lower than was required for the system 
adopted while at the same time the latter would permit the use 
of a single tube for both tracks instead of a separate track for 
each. 

At each side of the river and running out to a certain distance 
from the banks the tunnel was built or driven by the aid of a 



I 



DIVING BELL AND CAISSON 



283 



compressed-air shield and then caissons were sunk in the bed of 
the river to the required depth and in correct positions to form 
a continuous tube. There were three of these tunnel caisson 











I 

1 

D 

_^1 





Fig. 90. — End View of Tunnel Caisson. 



sections in the large arm of the river and two sections in the 
narrower arm. 

The halftones show quite clearly the construction of one of 
these caissons, some idea of the size of which may be formed by 
comparing with the men employed around it. The structure 



284 



COMPRESSED AIR PRACTICE 



comprised two principal parts, the tunnel proper and an outer 
framework which formed the actual caisson. The tunnel por- 
tion was approximately elliptical in section and composed of 
iron rings. Each ring was made up of a series of cast-iron 
voussoirs bolted together and the rings then bolted to each 
other with layers of treated wood between for a water-tight 
packing. 

Around this elliptical tube was built a metallic shell which in 
the lower part of it formed the rectangular caisson. The frame 
of this shell was formed of a series of curved ribs of channel iron 
outside the tunnel tube and braced at a certain uniform dis- 
tance from and nearly parallel with it by cross-braced ironwork. 
These ribs did not run around the under side of the elliptical 
tube, but, passing the center or horizontal axis, they extended 
vertically downward thus forming the sides of the caisson and 
extending some distance below the bottom of the tunnel they thus 
formed the compressed-air chamber in which the men worked 
to make the necessary excavation which allowed the whole 
structure to sink to the required depth. Over the ribs was 
placed a sheet iron covering which formed an external air-tight 
shell, the ends also of the caisson being thus closed. 

Each caisson was floated and towed to its place, piles having 
been driven there to guide it vertically during the sinking 
operation. The semi-elliptical space between the tunnel tube 
and the sheet-iron sides, all the central horizontal plane, was 
filled with cement baton, the top plating having been left off 
for the purpose. As the ground was excavated within and under 
the caisson by the ^' sand-hogs" it was gradually lowered to its 
proper level below the river bed, the tunnel tube having been 
ballasted with water to give the needed weight and steadiness. 
When finally in place the caisson below was filled solid with 
concrete, the shafts at the side, with the air locks for the passage 
of men and material, were removed, and after all the connections 
were made the water was pumped out of the tunnel tube. The 
ends of the caissons lay about five feet apart, and this space was 
filled by a small caisson which was sunk last. The removal of 
the end plates of all the sections left the continuous tube clear. 

There was a double pneumatic interest in this undertaking, as 
compressed air was employed not only in the sinking of the 
caissons but also in their construction, both of the halftones 
showing pneumatic riveters in use upon the frames and shells. 



DIVING BELL AND CAISSON 285 

The Submarine Diver. — The so-called '^ armor'' worn by the 
individual diver, giving freedom of movement under water, for 
doing all kinds of submarine work is still another application of 
the inverted-tumbler principle, although the original shape of 
the device is entirely lost sight of. The diver is clothed in a 
water-tight suit and then air is forced into the helmet portion 
at sufficient pressure to balance that of the water. In this case 
also no attention is required as to the maintenance of the proper 
pressure, only a sufficient and constant influx of air being required, 
the excess escaping and the pressure within automatically 
adjusting itself to the pressure without. The inflow and the 
escape of the air go on continuously, so that the diver will have 
fresh air to breathe. If the diver changes his position vertically 
the rate of escape of the air will vary accordingly, being more 
rapid when he ascends and less rapid when he descends, all his 
movements naturally being slow and deliberate. 

The diver's armor as now used has developed many refine- 
ments promotive of safety, of comfort and of facility and rapidity 
in working. The electric light and the telephone being now both 
available to the diver, he can do many things which his prede- 
cessors could never have thought of doing. 

Raising Ships 

The bouyancy of the inverted tumbler suggests the use of air 
for raising ships and it has often been successfully employed 
for this purpose when vessels have been at workable depths. 
A ship's hull floats because it has large empty spaces and when 
these fill with water she inevitably sinks, but if the water can be 
expelled she will rise again. When in a submerged vessel the 
sides and the deck are air-tight or can be made so by divers, if 
air can be forced in in sufficient quantity to expel the water, 
the vessel must rise no matter what may be the condition of the 
bottom. A most necessary precaution in such a case, and often 
a difficult one, must be to see that the entire ship rises together. 
It will not do to have the air accumulate and create a great pre- 
ponderance of buoyancy at either end. There must be some 
central partition or other means of distributing the air equitably 
to each end so that both ends will rise. When once afloat on 
even keel, there is no longer much difficulty in keeping a vessel 
righted and towing her to dock or elsewhere. 



286 COMPRESSED AIR PRACTICE 

Although compressed air has recorded a number of notable 
successes in the raising of sunken ships, its opportunities in 
that field are necessarily few and infrequent from the fact that so 
so few ships sink in such places or at such depths as to make it 
possible for the air to do anything in the matter 

A vastly greater work for compressed air lies before it in the 
preventing of ships from sinking. Here its possibilities apply 
not to a very few but to many or to all ships. If the unsinkable 
ship is ever built, or if such a consummation is ever nearly ap- 
proached, it will never be without the employment of compressed 
air. Under compulsion increasing attention is being given to 
this matter. The sinking of the Titanic will never be forgot- 
ten until the repetition of such a catastrophe has been made 
impossible. 

The most important device for keeping modern ships afloat 
is the transverse bulkhead, and it is a feature of all ships now 
built. That the bulkhead has actually saved many ships is 
matter of record. Typical local examples, as we might call 
them, we carry in our memory, all of them on trans- Atlantic 
Hues. There was the Arizona in 1879, the ''Greyhound" of the 
Atlantic of that period; she struck an iceberg in November, 
not the iceberg season; her first bulkhead held and she was 
safe. Then a few years later the City of Berhn had a very 
similar mishap — if you call it a mishap — of ramming an iceberg 
in the season of ice. Her bow was smashed into a shapeless 
mass, as the writer remembers the sight of it, but her forward 
bulkhead saved her. In 1889 the City of Paris had a novel 
accident, her engine having punched a hole through the bottom, 
and she was saved by her after bulkhead. When her engine- 
room was flooded and the ship was pitching, and the water was 
swashing back and forth, there was an anxious time of watching, 
and questioning as to whether or not the life barrier would hold, 
and only after this had been hastily but effectually braced by 
timber on the other side was the anxiety relieved. 

Bulkheads have not always held in times of great stress, 
and disaster has resulted. It is the practice to fill the sections 
successively with water to see that they are watertight before 
the ship is launched, but this gives no sufficient assurance of 
strength, and it is not so easy to devise a test which should 
satisfy. 

While the coUision bulkhead is one of the simplest of devices 



DIVING BELL AND CAISSON 287 

as to function, not a thing apparently which any engineer should 
need to study over except as to strength required and other 
details of construction, still it is a fact that even this simple 
thing has been misapplied, and it may also be made to appear 
that the application of it which might be most valuable of 
all has never yet been made. 

Of the unwisdom of great engineers over a very simple thing 
there is the episode of the longitudinal bulkhead in evidence. 
We can see now easily enough how the longitudinal bulkhead 
works. If a rent is made in the side of a ship provided with a 
central longitudinal bulkhead, and if one side of the ship, or 
even a not very large portion of one side, fills with water while 
the other side is not filled the ship of course rolls over. Yet, 
the British Admiralty had to lose at least two great ironclads, 
the Captain and the Victoria, with a loss in those two of perhaps 
as many men as went down with the Titanic, before this was 
realized. 

The bulkheads upon which we rely so much for the saving 
of our ships from sinking — when they save them — are water- 
tight transverse partitions which divide the interior of the 
hull into sections with the result that if any one, or any two, 
or sometimes if any three of these sections fill with water as the 
result of a collision, or the striking of a rock or an iceberg, the 
ship will still keep afloat. It is understood by the writer that 
in the Titanic there were fifteen of these bulkheads, or sixteen 
separated sections. Any two adjacent bulkheads exercise their 
saving function not at all upon the section which they enclose 
between them but on the section adjoining this one on either 
side. The enclosed section may fill as it will and the bulkheads 
offer not the slightest hint of opposition. Yet why not? 

Indeed, to protect the section which any two bulkheads 
jointly enclose should be their most important and imperative 
service. If each section besides its two watertight, or, for 
our present purpose, airtight bulkheads, had also an airtight 
deck enclosing ^ above, with airtight hatches or doors, and if 
absolutely nothing more were done or provided, then if a hole 
however big were stove into or near the bottom so that the 
water would rush in the section could not he more than half filled 
with water. The air which normally filled the section at a 
pressure of 1 atmosphere could only be compressed to a 
pressure, as long as the ship floated, not exceeding 2 atmos- 



288 COMPRESSED AIR PRACTICE 

pheres, or 15 lb. to the square inch by a pressure gage, and to a 
volume not less than one-half of what it was originally, and 
there the water would have to stop. 

If now compressed air were forced into thafc half-filled section 
at the pressure required, not exceeding 15 lb. gage, the water 
would all be driven down and out, that is down to the top of 
the hole by which the water had been rushing in, and to then 
hold the water from again rising above that level it would only 
be necessary to maintain the air-pressure by forcing in just 
enough air to make up for whatever air leakage might occur. 
It would not be necessary to wait for the water to rise in the 
section before beginning to force in the air. This should be 
begun as soon as the water began to rush in, or as soon there- 
after as possible. No care would be required about the air 
except to supply it in sufficient quantity. Any excess of air 
would be driven out at the bottom with the water and no in- 
crease of pressure could be thrown back upon the compressor. 
This is how the air is disposed of when men are working in foun- 
dation caissons. The air must be continuously renewed so 
that it may continue breathable, but it is only necessary to 
continue forcing the air in; its discharge will take care of itself 

Any compartment of the Titanic between two bulkheads and 
beneath the lower deck may be assumed to have had a cubical 
content something like this: 90 ft., abeam; 50 ft., fore and aft; 
30 ft., deep = 135,000 cu. ft. A considerable portion of this space 
would be occupied by coal or cargo, so that we may assume a 
space of 100,000 cu. ft. to be filled, or, in addition to the normal 
atmospheric content of the compartment, there would be an 
equal additional volume of air to be supplied, and with air 
compressors having a free air capacity of 5000 cu. ft. per minute, 
the space would be filled and the water would be expelled in 20 
minutes, and the maximum power required, would be 250 h.p. 
It is not necessary here to go into the practical details of the 
arrangement suggested, but it would be well to have oil-engine- 
driven compressors, which can be started at a minute's notice, 
located upon an upper deck, preferably one at each end of the 
ship and connected to deliver into the same pipe system, with all 
necessary valves, etc. 

It would seem to be a somewhat strange and unaccountable 
thing that there could at this day exist an opportunity for the 
writer, or for any one else, to be calling attention to this matter. 



DIVING BELL AND CAISSON 289 

There is nothing in the way of originality of invention in the 
suggestion, no special ingenuity is required in the appHcation 
of it and there can be no question as to the results which would 
be secured. 

If even now the providing of watertight bulkheads is made 
compulsory under the law, and very properly so (although it 
should provide more adequately for having them strong and 
stiff enough) it would seem that the providing of airtight decks 
and hatches, so that any section may be made a pressure chamber 
and water-excluding, should be equally so. 

Although this suggested arrangement promises really an 
additional element of safety greater than that contributed by 
the watertight bulkhead itself when not assisted bj^ the 
compressed-air feature, it entails little additional expense. 
The deck must be there in any case, and such decks even now 
are practically airtight. Some attention should be given to the 
bracing or strengthening of them to withstand the upward pres- 
sure, and there would be the providing of the airtight hatches, 
and in some cases double doors or airlocks would be needed, 
and that is all. The latter suggestion recognizes the fact that 
it would be easily possible for men to work in the air thus 
employed to hold the water back, for making repairs or stopping 
leaks from the inside, for passing up supplies, etc., the pressure 
being much less than that in which men work in sinking foun- 
dation caissons or in driving subaqueous tunnels. 

Thus far our thought has been of ships in general, with the 
passenger ship most prominent; but ships of war are really most 
ready of all, and also most in need of the application. They 
have the bulkheads and they have the decks with few and small 
openings through them, all of which may easily be provided 
with means of prompt and ready closure, and then all that is 
needed is the air compressor. 

The arrangement of the airtight bulkheads and the airtight 
deck would not generally be sufficient to keep afloat a warship 
of the modern type without a compressor to expel the water 
from all the space enclosed on account of the low freeboard and 
the limited space enclosed. 

In connection with warships especially it is proper to mention 
another service which compressed air might render in some 
emergencies. By the controlling of the air pressure in the dif- 
ferent sections and a proper manipulation of the bulkhead doors 

21 



290 COMPRESSED AIR PRACTICE 

masses of water might be easily directed from one section to 
another for the purpose of keeping the ship on an even keel, thus 
serving to prevent or defer the settling endwise which usually 
hastens, if it be not the actual cause of a final catastrophe. 

The following, mostly abstracted from The Engineer, London, 
tells us of what is actually being done in the direction above 
indicated. The U. S. battleship Pennsylvania, also the Nevada 
and the Oklahoma, are being equipped with an installation for 
localizing and minimizing underwater injuries, under the direc- 
tion of Mr. W. W. Wotherspoon, who has had successful experi- 
ence in the raising of sunken ships by the aid of compressed air. 

Every man-of-war is subdivided, from a point above the 
water-line to her keelsons, into many hundred separate water- 
tight compartments, for the purpose of confining and localizing 
injuries. These divisions are connected to an extensive drain- 
age system, and ordinarily powerful pumps are counted on to 
overcome leakage. If a compartment filled, notwithstanding 
the pumps, and the invasion stopped there, the consequences 
might not be very serious; but, unfortunately, the pressure upon 
bulkheads and decks is often too great; hence they yield, and the 
neighboring compartments are then flooded, and soon, the ship 
slowly but surely careening or sinking. 

Mr. Wotherspoon subdivides a ship into successive strata or 
layers of compressed air zones, and he distributes these as the 
emergency arises so as to meet the requirements of each exigency. 
In other words, the pressure of the air on the opposing sides of 
decks or bulkheads is such that the resultant difference on the 
side of grea-ter pressure is a number of pounds less per square 
inch than would be the case if the compartment were filled with 
water and flanked only by normal atmospheric conditions. In 
this manner the structure of the damaged space is supported by 
the enveloping body of the ship throughout a pretty wide area, 
and this effectually guards against the yielding of the walls 
directly encompassing the damaged region. 

Mr. Wotherspoon having met the official objections which 
were raised, the armored cruiser North Carolina was allowed for 
a test installation. It was elected to connect the apparatus 
with something like 800 of the ship's water-tight compartments, 
narrowly limiting the weight of the devices employed. 

The underwater compartments of all men-of-war are forcibly 
ventilated to guard against the accumulation of gases and foul 



DIVING BELL AND CAISSON 



291 



air. This is effected by a double system of piping, one carrying 
fresh air into the compartment and the other supplying an 
outlet for the tainted exhaust. Running down below the water 
line as it does, all of this piping is subjected to pressure tests and 
must be equal to any tax which may be placed upon it by water 
pressure due to flooding, and therefore these conduits already in 
place stood ready to serve another purpose. By these channels 
Mr. Wotherspoon can lead compressed air to any of the com- 
partments, and the only additional apparatus needed for this 
work is a flexible attachment for effecting connection with a 
source of air supply. Originally this was the air compressors, 
but it is now intended to have air stored in reserve in sufficient 
quantity to help toward the immediate checking of any danger- 
ous admission of water. 

A supplemental feature is that of air locks extending from the 
uppermost water-tight deck down to the lowest water-tight 
spaces, and these air locks, by means of suitable vertical doors — 
not the top doors — on each deck make these convenient passage- 
ways between decks for all ordinary traffic. 




Fig. 91. — Compressed Air Chambers on Warships. 



The athwartship section. Fig. 91, shows the general arrange- 
ment of the system upon one of the United States naval vessels. 
Compartment A is the damaged one, and theoretically is under 
an air pressure of 14 lb. to the square inch. The contiguous 
compartments B B B are filled with air at a pressure of 9 lb. per 
square inch, and this leaves a resultant bursting pressure upon 
the walls of compartment A of only 5 lb. per square inch. The 
outlying spaces C C C are charged with air at a pressure of 4 lb. 



292 COMPRESSED AIR PRACTICE 

per square inch, and again this leaves a resultant bursting pres- 
sure of 5 lb. within compartments B B B. Therefore instead of 
the walls of the damaged compartment A having to withstand 
the entire stresses due to the confined air at 14 lb. pressure per 
square inch, the flanking and superposed enveloping ship fabric 
is bearing its part progressively. 

According to the building specifications of every fighting 
craft all of the water-tight compartments are required to be 
tested some time during the ship's construction. This test for 
water-tightness when carried out does not involve pressure test- 
ing to any pronounced extent, and the spaces are seldom, if ever, 
filled as they would in all probability be in case of flooding due to 
damage. In the case of the North Carolina it was found that 
more than one compartment was leaky when the compressed air 
was used. Not only was this so, but a good many water-tight 
doors theoretically perfect proved to be anything but tight. In 
most cases this was found to be due to worn gaskets and loosened 
fittings, but these potential leaks would have escaped notice but 
for the telltale hissing of the liberated air. Thus, apart from 
its virtue in case of injury to the bottom platings, the system as 
installed upon the North Carolina showed how valuable it could 
be in checking the water-tightness of the craft within its own 
body structure. The secondary usefulness might easily resolve 
itself into one of prime importance under some conditions. 

Mr. Wotherspoon has made his system valuable for the smoth- 
ering of fires as well as for the arresting of leakage. Instead of 
turning compressed air into a compartment he chokes out a con- 
flagration by using the same conduits to carry a gas which will 
not support combustion into the endangered division. 

A third advantage of this dual fire-and- water protecting system 
is the peculiar facility which it offers for making temporary 
repairs. By means of the air locks it is possible for the men to 
enter an affected compartment and to overhaul a leaky valve 
and replace it without any inconvenience. Again, should the 
injury involve a rupture of the ship's inner or outer plating, 
that, too, can be temporarily stopped or plugged or otherwise 
sealed as the occasion may best afford. The men would simply 
be working in the structural double of a caisson. 



CHAPTER XVII 
AIR JET— SAND BLAST— CEMENT GUN 

First it will be worth while to note what is done by the air 
blast alone. Dry, clean air, bearing no sand or other material 
is used for polishing small articles of metal, in this case the high 
velocity of the blast impinging upon the metal producing the 
polishing effect. In the centrifugal machine the load, whatever 
it may be is normally accompanied at the same speed by the 
air which fills the interstices. With the basket of the machine 
rotating at its maximum speed a jet of air is pointed to blow 
in the reverse direction, and this is found to have a polishing 
effect upon the contents. Nickel-plated articles, for instance 
which have become tarnished, and small articles generally which 
require a finishing touch are brightened up very quickly, and this 
has become an established practice in certain factories. It is 
only necessary to pack the articles, which are generally small, 
so that the air can attack them all over. It may not have been 
suggested, but it might be well without changing the loading to 
reverse both the direction of rotation and the pointing of the 
air jet. This would give the air a surer chance of getting at all 
sides of the articles to be polished. 

The Air Blast in Foundation Sinking. — A contracting company 
in New York City is making a specialty of the use of the air 
blast as an aid in sinking building foundations. In this work 
the foundations are sunk to rock which is overlaid by varying 
depths of gravel or other compacted material. A recent under- 
taking was the construction of the foundations for a large twelve 
story building where the rock was below the surface at depths 
ranging from 17 to 50 ft. When completed the foundation con- 
sists of a series of reinforced concrete structures, these being 
formed of groups of 12-in. steel pipe filled with concrete among 
which are distributed vertical reinforcing rods of steel. 

The pipes used are in lengths of 12 or 14 ft. and in the deepest 
parts three or four of these are required, one above the other. 
Each pipe after being properly located in a vertical position 

293 



294 COMPRESSED AIR PRACTICE 

is driven down a few feet by an air-operated pile-driving hammer 
placed upon the top of it. 

The pipe having been thus driven a certain distance the 
hammer is lifted off by a derrick, the pipe is filled with water, 
a compressed-air pipe is thrust down as far as it will go and 
also a test rod which is used to ascertain when the solid rock is 
reached. 

On the street close to the work is located a 20-in. steam actuated 
air-compressor and a large receiver. The compressor is speeded 
up and when the receiver is filled to the highest working pressure 
all the air is suddenly turned into the foundation pipe, blowing 
out the water, the dirt and the stones. This filling with water 
and blowing out is repeated two or three times if necessary 
until the air pipe has been sunk to the bottom of the casing 
pipe. Then the hammer is put to work again, the pipe is driven 
down another couple of feet or so, then it is filled with water 
and blown out again until the rock is reached. The pipe is 
even finally driven a foot or more into the rock, the familiar 
New York mica-schist. 

When the driving for the individual pipe is finished several 
steel reenforcing rods are inserted and then both the enclosing 
pipe and the rods are cut off to the required level by the oxy- 
acetylene torch, the pipe is filled with concrete and finally capped. 

This method is also being successfully employed for putting 
in new foundations under old buildings, a section of the building 
being cut out to give room for a short length of pipe and a hy- 
draulic jack, the pipe being alternately jacked down and blown 
out and followed by additional lengths of pipe until the required 
depth is reached. 

The Sand Blast. — In the case of the sand blast the air jet is 
made to carry an abrading material and to deliver it with such 
force that the material has a sharp effect upon the surfaces by 
which it is arrested. It is not easy to name any specific employ- 
ment of the sand blast which may be called its principal work. 
For the cleaning of castings it may be said to be used in every 
enterprising and up to date foundry, so that in this line it is not 
only numerously employed but also the air is used in consider- 
able volume in each case. 

In the progressive extension of the use of the sand blast the 
tendency, as in similar cases, has been toward simplification of 
apparatus, some of the changes resulting perhaps in an apparent 



AIR JET— SAND BLAST— CEMENT GUN 295 

sacrifice of ecomony in the use of the air. The use of a high 
pressure for the air rather than a lower, or vice versa, can scarcely 
be regarded as a simplification one way or the other. There 
is still more or less discussion as to whether low pressure — 15 or 
20 lb. — or high pressure — 80 to 90 lb. — is to be preferred. 

The low-pressure advocates recommend the use of a pressure 
reducer and an auxiliary air-receiver near the work when the 
supply is taken from a high-pressure service. The low-pressure 
system will use larger nozzles and cover a greater area of work 
at a time, but the sand will not impinge with as great force as 
with the high pressure. 

Both the sand and the air must be dry, the latter, of course, 
not absolutely so, and some knowledge of the means of securing 
air comparatively dry will be likely to leak into the mind of any 
one who becomes at all familiar with the present volume. 
Sand in foundry practice is used over and over, and if it becomes 
damp so that it will not flow with perfect freedom it will require 
to be dried in an oven or otherwise. There are various effective 
sand driers in the market. 

Sand should be clean and sharp and for the larger nozzles it 
need not be as fine, and indeed is better not to be as fine as for 
the smaller nozzles. The sand does all its work by the sudden- 
ness of its impact upon the metal, so that it is more or less shat- 
tered into smaller fragments or dust, which becomes useless and 
must be sifted. Sand for the sand blast is being manufactured 
in different grades by the crushing of suitable rock and sifting 
to the required sizes. 

Different pressures are recommended for different kinds of 
work, the requirements for ordinary castings ranging from 5 to 
20 lb., while for steel castings as high as 75 lb. is recommended. 

The sand blast is used with various arrangements looking to 
expediting the work or protecting the workman. Fixed jets 
are often used to deliver the sand blast directly upon the contents 
of revolving tumbling barrels. A slowly but continuously 
revolving turntable has been employed with a heavy leather 
curtain suspended across the center, one-half of the table being 
exposed to several fixed blast nozzles which will effectively 
clean the castings and as these emerged from behind the curtain 
they can be successively removed and replaced by others. 

In one case the sand blast is on one side of a tight partition and 
the workman on the other side reaching his glovo-protected 



296 COMPRESSED AIR PRACTICE 

hands through holes in the partition for directing the blast or 
manipulating the castings and looking through panes of glass 
placed at a convenient height. In the place where this arrange- 
ment was in use it was necessary to replace the panes of glass 
every day. 

The rapid wear of the nozzles is the most troublesome thing 
about the sand blast. The hardest steel needs frequent renewing 
being finally discarded because the bore wears too large. The 
best thing known to the writer is so-called ''white" iron, or the 
iron of which malleable iron castings are made but without the 
''annealing/' or rather the carbonizing process. These castings 
are so hard that they cannot be machined or even ground at all, 
and must be used just as they come from the sand. They cannot 
be cored much less than 3/8 in., but we have seen many such used 
for rough work, say about 1 in. in diameter outside and 6 in. long. 
These are easily connected to a rubber hose. 

The sand blast is frequently used for cleaning the outsides of 
stone and other buildings, and as only light pressures are required, 
varying with the height at which the work is carried on, the air 
can be supplied in sufficient volume by an electric-driven or a 
gasoline- driven portable compressor, and apparatus of this 
type is not unfamiliar in the large cities. 

The following concerning the use of the sand blast in gold 
and silver art work is a brief abstract of a portion of a valuable 
paper by Frank Mason of the University of Sheffield. It is 
not only intrinsically interesting, but it is more than that, sug- 
gestive of the many other possible uses of the sand blast for the 
delicate manipulation of surfaces of wood, glass, metals, etc. 

The continued demand for variety in the finish and appear- 
ance of gold, silver, and electro-plated goods, gives to the sand- 
blast such a wide field of operations as to be almost unlim- 
ited, and the opportunities, now opening up, for its application are 
without parallel in its history. As a ready means of imparting 
to silver and other materials, at a small cost, a finish very pleas- 
ing to the eye, it is hard to rival. From the number of machines 
specially built for different classes of work, constantly being placed 
on the market, it would seem that blasting- machine manufacturers 
are alive to the possibilities of their productions. These are now 
so numerous that to select the most suitable w^ould be almost 
impossible without first being acquainted with the class of work 
to be done. The two chief governing factors of the process are 



AIR JET—SAND BLAST—CEMENT GUN 297 

the pressure and the material used. Obviously, therefore, the 
method of procedure may be varied in two directions. 

1. By varying the pressure used in forcing the blasting material 
against the article. 

2. By using material with varying cutting properties and of 
different grain. 

A matte may be obtained in the former case, very deep or 
slight, but not necessarily coarse, even under a pressure of 18 to 
20 lb. per square inch. This factor (the power), then governs to 
a large extent the depth of the matte and varies between about 
2 to 20 lb. per square inch, according to class of work and 
desired ultimate appearance. The question of blasting material 
is one of undoubted importance and yet, strange to say, is very 
often overlooked. A visit to a continental plate and jewelry 
establishment would very quickly convince the sand blaster of 
the obvious difficulties he would encounter in endeavoring to 
produce such surfaces as he would see displayed in a first- 
class house. The peculiarly frosted, delicate, French gray 
finish so dear to the artistic Parisienne, and to be seen all over 
the continent is a striking example of the careful manipulation 
and the delicate treatment necessary in some sand-blasting 
operations. In this class of work this process is the last to which 
the article is submitted, hence its requirement of very careful 
treatment, judicious selection of blasting material and well- 
regulated low pressure. 

Another instance in which very careful manipulation is required 
is in the production of the lovely, soft, greenish gold tint so much 
admired. This is obtained on gold-plated articles, as well as 
those manufactured wholly from the metal itself. Here again 
under even a moderate pressure from the sand blast, the deposit 
of gold might be completely spoiled. The method adopted in 
most cases to obtain the above finish is first to gild or gold-plate 
the articles, bringing them from the gold bath a little darker in 
color than is required in the finished appearance. If heavily 
coated, scratch brush, preferably with very fine German silver 
wire brush, and blast with flour of pumice under a pressure not 
exceeding 3 lb. per square inch. Wash away any pumice 
clinging to the article and wipe with very soft chamois leather. 

A very fine surface appearance, usually employed on articles 
having parts in high relief, is obtained by means of the sand blast 
on ** oxidized" silver as follows: The process of sand blasting 



298 COMPRESSED AIR PRACTICE 

should be operated just prior to silver-plating, using powdered 
pumice and a pressure of about 12 lb. After completing the 
deposit, ^'oxidize" same in a solution of potassium sulphide until 
the article assumes a rich deep blue, or blue back color. Then 
by means of a calico mop or dolly and Trent sand, relieve where 
necessary according to design. The result of this process is 
quite a study in light and shade and is productive of some very 
fine effects. Satin finish, now often produced by means of the 
sand blast, is a term used to indicate the appearance of articles 
bearing a matted surface. 

Crystalline, ice-like surfaces similar to some molded glass 
wares, are sometimes desired. It is necessary in these cases to 
employ a high pressure, say 18 lb. per square inch. As blasting 
material, a coarsely ground glass or very coarse sand should be 
used. This, of course, is done before plating, and as it is quick 
and severe in operation must not be overdone by prolonged 
blasting. '^ Partial frosting," as the term suggests, is the pro- 
duction on one surface of a combination of matted and brilliantly 
burnished parts. 

This gives to the article a very pleasing contrast and is im- 
parted thereto by means of the stencil. It may be accomplished 
very readily by cutting from ordinary writing paper or similar 
material the pattern or parts to be left bright or burnished, and 
then glue same to the article. Allow to set thoroughly, and 
sand blast unprotected portions. The glued paper is easily 
removed by suspending the article in hot water. Subsequent 
processes, such as plating, etc., may then be proceeded with. 

The Sand Blast for the Bath. — The last refinement of the sand 
blast is perhaps in connection with the bath, where it has been 
employed to give the last dainty touch to the human form 
divine, the operation being somewhat Frenchily sketched as 
follows. 

''A few bushels of sand is brought to her room and after being shghtly 
warmed it is spread out upon a sheet. A maid rubs her body all over 
with fine sandpaper, and after this process soft, rich cream is massaged 
into the skin. Then the bather stands in the middle of the sheet and 
taking up handfuls of the sand she rubs it over her body until she is 
glowing with the friction. 

"Then she reclines at full length on the sheet, the ends of which are 
folded over her, and rests several moments before rolling over and over 
so as to become completely immersed in the sand. Then follows the 



AIR JET— SAND BLAST— CEMENT GUN 299 

'blow-bath.' From a sort of fan-shaped blower sand is whirled out 
briskly so as to strike the body as forcibly as the bather can stand it. 
The effect is said to be wonderfully stimulating, and the sting is not 
unlike that produced by electricity." 

Pneumatic Painting. — From the sand blast, conveying and 
forcibly discharging comminuted solids to impinge upon and 
abrade the surfaces with which it may come in contact, is an 
easy step to the conveyance and discharge of a true liquid 
instead of the sand, and then we have the various paint-spraying 
devices. These have in many cases been savers of labor where 
large surfaces were to be quickly and cheaply covered, their work 
being more satisfactory for whitewashing or kalsomining rather 
than for painting proper, although for the latter purpose pneu- 
matic devices have been extensively used for painting freight 
cars and similar work. In this the results can scarcely be ranked 
very high, and are nothing for compressed air to boast of. As 
with the sand blast, however, pneumatic painting also has its 
dainty touches in some of the details of art work. 

The Cement Gun. — An apparently important and a com- 
paratively recent application of compressed air in the cement 
gun may be said to be a combination of the sand blast and the 
paint sprayer. The success or even the practicability of the 
cement gum could scarcely have been regarded with much con- 
fidence until demonstrated by actual experiment. There were 
two important questions which abstract thought could never 
have settled. The value and reliability of cement or concrete, 
assuming that the individual ingredients are what they should 
be, depends upon the maintenance of the correct proportions in 
the mixture and especially the quantity of water; and then as 
when the ''gun" was used the cement was to be applied to sur- 
faces at every angle from horizontal to vertical and from vertical 
to all the angles above the vertical to horizontal overhead. 

Both questions are practically and most satisfactorily answered. 
In the handling of cement mortar in the ordinary way there is 
a theoretical proportion of water that is best for the material 
when set ; but this theoretical amount of water forms a mortar 
too stiff to handle, while an excess of water weakens the mass 
by causing voids when the water has disappeared. In the 
cement-gun process it is claimed that no excess of water can 
remain in the mortar to later cause voids neither is it possible 
for the finest particles of the cement to begin to set before the 



300 



COMPRESSED AIR PRACTICE 



mortar is placed. The cement gun appropriates only the neces- 
sary amount of water for the proper hydration of the cement, 
and as the materials are projected with considerable force all 
surplus water and air are expelled. The product is a non- 
porous, impermeable mass possessing the maximum density and 
practically waterproof. 

As to the adhesion of the cement we have the following ex- 
position from a paper by Mr. William A. Gordon in Transactions 
of American Society of Engineering Contractors: 

If a sand blast is directed against a surface of a comparatively 
soft and sticky nature, such as ordinary wax, a very curious and 
unexpected thing happens. The particles of sand, instead of 
instantly tearing and wearing the wax away, as might be expected, 
penetrate the surface and stick, thus becoming a part of the mass 
itself. This process is continued until the wax is, as it were, 
saturated, and no more sand can find lodgment therein; in other 
words, the wax arms itself against attack with the sand itself, 
and thus the most powerful sand blast is rendered harmless. 

Let us consider now what might be expected to happen if a 
certain proportion of powdered wax were introduced into the 
air blast with the sand, and the combined material directed 
against some hard surface, a sheet of boiler iron, for instance. 
Of course, the first particles of sand would strike the iron and 
bounce off, much as a pebble does when thrown against a wall; 
but, being soft and sticky, every atom of wax would adhere, and 
within a few moments the entire surface of the iron would be 
protected by a thin coating of wax. After this coating had be- 
come thick enough to enable the grains of sand to embed them- 
selves therein, they would cease to bounce off as at first, but would 
stick and become a part of the mass or coating itself. It is easy 
to understand that, if the process were continued, the sand and 
wax could be made to form as thick a coating on the surface 
of the iron as might be desired. 

If in the above experiment in place of wax we introduce 
cement and water, the operation of the cement gun is easily 
comprehended. When the nozzle is first directed against any 
hard surface, the particles of sand do not at first adhere; they 
fall away until a coating of cement sufficiently thick is formed to 
enable the sand to embed itself. When the cement gun is apply- 
ing new cement to a body which has already set, the effect in 
practice is to deposit first a thin layer of practically neat cement 



AIR JET— -SAND BLAST—CEMENT GUN 301 

where the new work joins the old, and after hardening if a test 
piece is broken it will be found that this joint or initial surface 
of adhesion is the strongest part of the mass. 

The cement gun came, as we might say, from the outside, and 
was not first thought of, tried and brought to success by any 
engineer, builder or contractor. It was originally conceived by 
Mr. C. F. Akeley, a taxidermist of Chicago. His idea was to 
use the device to rapidly and economically build up the forms 
upon which to mount the skins of large animals, the promised 
advantage being that material could be added locally little by 
little to the limbs or bodies as judgment dictated, and it was a 
pronounced success at once. 

It happened that Mr. Akeley was a member of the Field Mu- 
seum committee, and had charge of the remodeling and preserv- 
ing of one of the old World's Fair buildings in Jackson Park, 
Chicago, which had been given to the Field Museum Associa- 
tion. He produced an enlarged cement gun and employed it in 
covering the entire building with stucco work. The experiment 
was entirely successful, the work was done rapidly, well within 
the estimated cost, and the life work of the cement gun had 
commenced. 

A brief description of the cement-gun apparatus and mode of 
operation is all that can be given here. It is generally neces- 
sarily portable, in the smaller installations the mixer carriage 
having an air-compressor and appurtenances upon it, making 
the arrangement entirely self-contained, while for work of larger 
and more extensive character the air supply is separate from the 
'^gun" machine proper. 

A mount of the latter type is sketched in Fig. 92, the air- 
supply coming from some other source and connected by suitable 
hose. A vertical section of the entire machine is shown with a 
reduced section of the mixing apparatus and also of a typical 
delivery nozzle. 

The sand and cement, mixed in correct proportions and per- 
fectly dry, is dumped into the upper chamber A through the 
swinging gate C which closes against a rubber gasket. Chamber 
B is separated from chamber A by a similar swinging gate D 
with its rubber gasket. The arrangement of these two chambers 
constitutes an air-lock which permits the intermittent insertion 
of material without interfering with the continuous operating 
of the gun. In the lower parts of A and B are vertically rotated 



302 



COMPRESSED AIR PRACTICE 



agitators P, which are operated by hand. These are for break- 
ing up any lumps of the mixture of cement and sand which might 
be overlooked by the charger. 

The feed or discharge apparatus is ingenious and interesting. 
At the very bottom of B and below P is the main agitator or feed 
wheel L, which is operated by an air motor. This wheel is a 
solid steel plate about 1 in. thick with the edge notched all around, 
making it look like a cog wheel. The pockets in the edge formed 
by these serrations carry the dry mixture in rapid intermittent 




S^^/ L 




Cement Gun Apparatus. 



charges over the ''blow-pipe" I and its charge of compressed air, 
thus the mixture is blown upward and outward through the goose 
neck H into the flexible hose on the end of which is the nozzle. 
The driving air enters through the pipe S. 

The gun is operated as follows: the gate C is open with gate 
D closed, at which time it is possible to run the air through B 
to test out the hose and nozzle, at the same time pouring a charge 
of the dry mixture in A . The gate C is now closed and the three- 
way valve in the equalizing pipe opened, letting the com- 



AIR JET—SAND BLAST— CEMENT GUN 303 

pressed air into A. In a short time the air-pressure will hold C 
closed; a few seconds after this the pressure in A equals the pres- 
sure in B, and the gate D, which has been held in place by the 
air-pressure, opens automatically, due to the weight of the charge 
on the top and the equal air-pressure on both sides, permitting 
the charge in A to fall into B. Next the gate D is closed by hand 
and the air exhausted from A, when the gate C opens by its own 
weight. The air in A only has the duty of keeping C closed so 
that the pressure in B remains constant, assuring a uniform dis- 
charge from the feed hose. The gun is now ready for another 
charge of dry mixture. 

The feed wheel L revolves on a center vertical shaft and is 
propelled by a small air driven-motor, direct-connected by 
bevel gears. As this wheel revolves, it carries a small amount 
of the dry mixture at a time directly into the main compressed- 
air current. / and H are hooded so that the current carries 
directly from the former to the latter, picking up the small 
charge of materials on the way. The output of the gun depends 
upon the speed of L, and this 'can be governed by a valve on the 
supply side of the air motor. Thus it is that the gun has a con- 
tinuous feed, and is, therefore, continuous at the nozzle. To 
stop the stream of materials, simply shut G and the air to the 
motor. The bypass F and its valve is used to blow out the hose 
if it should clog. The air for shooting is admitted at S and on 
this line just beyond G is an air-pressure gage. The feed hose 
with the dry materials leads off from R to the nozzle. 

The nozzle is of particular interest, as it is there the dry mix- 
ture first comes in contact with the water for hydration. The 
water under ordinary city pressure, or about 35 lb. per square 
inch, is delivered to the nozzle through a 1/2-in. flexible hose, 
which is entirely separate from the feed hose. At the larger end 
of the nozzle is a water sleeve B, completely around it, so that the 
water enters as a fine spray from eight 1/16-in. holes at C, from all 
sides impinging toward the center. The amount of water used 
is regulated by a valve at the nozzle. At first a great deal of 
trouble arose due to the wearing out, by the abrasive action 
of the sand and cement, of the smaller end of the nozzle, so now 
a rubber lining A is used, which holds up better than any sub- 
stance yet tried. 

The materials, in a wet state, leave the nozzle at about the 
rate of 300 ft. per second. From this fact the "gunite" is far 



304 



COMPRESSED AIR PRACTICE 



denser than hand-applied, also of a greater tensile strength. A 
test which helps to bear this out was made. One ton of cement 
mortar applied by hand covered 25 sq. yd., 1 in. thick, while the 
gun covered only 16 1/2 sq. yd. 1 in. thick. 






ii 



CHAPTER XXVIII 
LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 

Air, like water, may exist either as a solid, as a liquid or as a 
vapor or gas. Water is most familiar to us in the liquid state, 
and this we think of as its normal condition. By artificial 
changes of temperature alone we convert it into a vapor. On 
account of association and familiarity we consider the normal 
state of the air to be the gaseous, but we have learned by changes 
of temperature and pressure, in the reverse direction, to convert 
it into a hquid or a solid. 

The boiling-point of air, the liquid, at atmospheric pressure 
is— 312°, or 524° below the atmospheric boiling-point of water. 
As with water, the boiling-point of liquid air varies with the 
pressure. At a pressure of 294 lb. the boiling-point is —240° 
or 72° above the atmospheric boiling-point. At a pressure of 
573 lb. the air changes from the liquid to the gaseous state, or 
vice versa, at a temperature of —220°. This is called the critical 
temperature of air, and no pressure, however great, can cause 
it to liquefy at a temperature higher than this. 

For every vaporizable liquid there is a certain temperature 
and pressure at which it may be converted into the vaporous 
state in the same space which it occupies as a liquid, the tem- 
perature being the dominating condition, and when above this 
critical temperature a gas, whether a true gas, or a mixture of 
gases, which the air is, can be compressed down to the liquid 
volume of its mass without liquefying. 

Liquid air is somewhat lighter than water, its specific gravity 
being . 94. When liquid air is confined and alllowed to evapo- 
rate at ordinary atmospheric temperature it generates a pressure 
of about 12,000 lb. to the square inch. The relative volume of 
free air at atmospheric temperature and pressure as compared 
with it in its liquid state is about 800 to 1. 

It must be confessed that cheap liquid air has been more or 
less of a disappointment to many engineers, and to the general 
public who have not been well informed as to its properties and 
22 305 



306 COMPRESSED AIR PRACTICE 

possibilities. The impossibility of retaining it continuously in 
the liquid state was the first trouble; it boils away rapidly in 
spite of all that can be done to insulate it. Its low temperature 
at once suggested its employment for refrigerative purposes; 
but the commercially established refrigerative processes do the 
work so much more cheaply as to render the proposition almost 
an absurdity. Those who have tried to use the reevaporating air 
for power purposes have had no better success, and the liquid air 
mine rescue apparatus has also demonstrated its impracticability. 

In another direction of utilization, however, liquid air has 
already accomplished wonders, and its utilization in this direc- 
tion is growing rapidly. This is as a source of oxygen supply 
for industrial and other uses, and in this field alone an enormous 
business has developed, the cheapness and readiness with which 
oxygen may now be supplied having led to the rapid growth of 
industrial processes of the highest practical value. The recent 
wonderful achievements in metal welding and cutting, as by 
the oxy-acetylene, oxy-benz and similar processes are all de- 
velopments following the cheap production of liquid air. 

After air is liquefied, if it then is allowed to reevaporate the 
evaporation differs in an important particular from that of water. 
It is generally understood that the two principal gaseous constit- 
uents of water, oxygen and hydrogen, are united chemically, so 
that water may be evaporated and condensed back again, and 
the operation may be repeated over and over again and the 
water will continue to be water as at the beginning. 

In the case of liquid air, however, this does not hold true. 
The two constituent gases, oxygen and nitrogen, do not so truly 
and constantly adhere to each other. The gases have different 
boiling-points, the nitrogen boiling away before the oxygen, so 
that there is provided a ready means of separating the two gases 
or of abstracting either of them individually and separately 
from the atmosphere to be used for any service that may develop 
for them. 

Professor Carl von Linde succeeded in liquefying air in a 
commercially practical manner in 1895, and the possibility of 
the cheap production of oxygen was recognized almost at once. 
This has been developed into a great business already, the British 
Oxygen Company, employing the Linde processes, having eight 
large plants in different^cities of Great Britain with an aggregate 
capacity of 400,000 cu. ft. of oxygen per day. The Linde Air 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 307 

Products Company in the United States also has extensive plants 
in many cities, while Germany and other European nations are 
also active in the same direction. 

Since 1895 there have of course been many improvements in 
methods and apparatus employed. In the earlier manipulations 
only about two-thirds of the oxygen could be abstracted from the 
air handled, but by the refinements of process developed only 
recently by M. Georges Claude nearly all the oxygen is secured. 
The entire matter has recently been presented to the public in a 
valuable unsigned article in The Engineer , London, April 4 and 



qMM 




Fig. 93. — Linde Liquid-air Plant. 



11, 1913, and what follows here is mostly abstracted from that 
article. 

By the Linde methods the expensive laboratory processes for 
producing the low temperatures required were rendered obso- 
lete, and a self -intensive procedure was substituted. Where a gas 
under pressure is allowed to expand through an orifice to a 
lower pressure without doing external work, the final tempera- 
ture is slightly lower than the initial. For air initially at 63° the 
fall is . 46° per atmosphere difference of pressure, so that if the 
expansion were from 11 atmospheres to 1 atmosphere the drop 
of temperature would be 4.6°. Linde used this apparently 



308 COMPRESSED AIR PRACTICE 

small cooling effect in a way to make it cumulative and so pro- 
duced the low temperature required. 

Air Liquefaction. — Fig. 93 is a diagram showing the essential 
features of the apparatus. Air compressed in cylinder A to, 
say, 75 atmospheres was passed through a water jacket cooler 
B, and its temperature was reduced to about 50°. On leaving 
this cooler the air traveled down the inner tube of a '^ reverse flow 
cooler" C and thence through a throttle valve D into a receiver 
E. With 75 atmospheres pressure above the throttle and 25 
below it the free expansion of the air through the valve D lowered 
the temperature of the air by about 23°. 

This cold air leaving the receiver passed by way of pipe F 
into the outer jacket of the reverse flow center. While flowing 
up this jacket it abstracted some of the heat from the succeeding 
charge of air flowing down the inner tube. If the interchange 
were completely effected the first charge of air would leave the 
reverse flow cooler by way of the pipe G at 50°, the temperature, 
that is, of the air leaving the water jacket cooler B . On the other 
hand, the second charge of air would reach the top side of the 
throttle valve at 27°, the temperature, that is, of the first charge 
of air after it had passed the throttle valve. 

The first charge of air flowing through the pipe G was again 
compressed in the cylinder A and again cooled to 50° in the 
jacket cooler B. The second charge of air had by this time 
expanded through the throttle valve D, and its initial tempera- 
ture of 27° had thereby become reduced about 29°, so that in 
the receiver its temperature would be —2°. The first charge 
was then cooled to — 2° by the second charge in the reverse flow 
cooler, and the second charge again brought up to 50° and 
returned to the compressor. As this cycle was repeated each 
charge of air arrived at the throttle valve at a successively lower 
temperature until the critical temperature, —220°, was reached. 

At this point the air, so far as temperature was concerned, 
was in a condition to be liquefied. The critical pressure for air 
at that temperature is, however, over 40 atmospheres, and, as 
the pressure in the receiver was only 25 atmospheres it is clear 
that even with the air at the critical temperature it would still 
be in the gaseous state. The cooling cycle was therefore carried 
still farther until the temperature of the air in the receiver was 
sufficiently low to enable the pressure of 25 atmospheres to pro- 
duce liquefaction. For convenience the air has here been spoken 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 309 

of as in successive charges, but in practice the process was really 
continuous, and as liquid air was drawn off at H, fresh atmos- 
pheric air was introduced at J. 

The Separation of the Oxygen. — Under ordinary atmospheric 
pressure the boiling-point of liquid nitrogen is —296.5°, and that 
of hquid oxygen is —320°, a difference of 23.5°, and it would 
seem a simple thing to boil away the nitrogen and retain the 
oxygen by keeping the liquid below its boihng-point; but it did 
not work out in that way. 

It was quite practicable to boil off the nitrogen until the re- 
maining liquid contained 60 per cent, of oxygen, but beyond 




Fig. 94. — Linde Oxygen Plant. 



that more and more oxygen was carried off with the nitrogen 
until the quantity of pure oxygen ultimately remaining was only 
a small fraction of the quantity present in the original volume 
of air. Not until 1902 could the rectification of liquid air be 
performed on a commercial basis. By Linde's method then 
devised he was able to extract no less than 93 per cent, of the 
oxygen from a given quantity of air, and to deliver it with a 
purity of about 99 per cent. 

Referring now to Fig. 94, it is first to be supposed that the 
apparatus is just started up for a fresh run. Valves A, B, and 
C are closed, and air, compressed and cooled as before described, 



310 



COMPRESSED AIR PRACTICE 



is entering the system at D. Flowing down the center pipe of 
the reverse flow cooler E, it passes through a worm F lying 
within a receiver G. Leaving the worm, the compressed air 
reaches the throttle valve H, where expanding to a lower pressure 
it is cooled thereby as before. The air thus cooled ascending 
pipe / is led to the top of the rectification column or chamber. 
Here it is sucked into pipe K, whence it is conducted through 
pipe L and the outer jacket of the reverse flow cooler E back to 
the compressor by way of pipe M. 

With the exception of the worm F, the scheme is the same as in 
Fig. 93, so that after the apparatus has been working long enough 
the air leaving pipe / begins to carry with it an increasing 
quantity of Hquid. Such Hquid air, instead of passing over into 
pipe K, will fall down the rectification column and collect 
around worm F in receiver G. 

We may suppose that the apparatus has been working long 
enough to have worm F completely immersed in hquid air. 
Valves A, B and C are now opened so as to bring a second 
reverse flow cooler N into parallel working with the first. The 
air from the compressor can thus flow through both coolers E and 
N on its way to the worm F. When it reaches this worm it is 
cooled by the surrounding liquid air, and on expanding through 
the throttle valve it emerges from pipe J in the Hquid form. 

As the compressor is now drawing direct from the atmosphere, 
a continuous stream of liquid air is falling down the rectifier 
from the pipe J. But if the air inside worm F is being cooled 
by the surrounding hquid air the latter must thereby be warmed. 
There is then passing down the rectifier liquid air, and passing 
upward is the cold gaseous air reevaporated within the receiver 
G. The two streams are brought into intimate contact by means 
of perforated plate baffles arranged within the rectification 
column. 

The vapors ascending the rectifier are at first very largely 
nitrogen. These nitrogenous vapors are at a sensibly higher 
temperature than the down coming current of liquid air, so that 
a transfer of heat takes place between the two currents. The 
comparatively hot nitrogenous vapors evaporate again some of 
the descending hquid air, this evaporation being accompanied 
by a predominance of nitrogen in the evaporate. 

During these earlier stages of the working the original hquid 
air in the receiver G is from a two-fold cause becoming richer in 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 311 

oxygen and poorer in nitrogen. Ultimately the liquid surround- 
ing the worm F will be practically pure oxygen, and when this 
stage is reached the apparatus begins to fulfil its function of 
producing oxygen, and incidentally nitrogen, as comparatively 
pure gases at ordinary atmospheric temperature. 

The Hquid air which flows out of pipe J in descending over 
the plates in the rectification column is brought into intimate 
contact with the ascending gaseous oxygen which itself has 
evaporated while passing through the worm. The cold liquid 
air and the slightly hotter gaseous oxygen have a transfer of 
heat, and the ascending gaseous oxygen is cooled and condensed 
and falls back into the receiver as liquid. The descending 
liquid air is heated; the more volatile constituent, nitrogen, is 
boiled off and ascends the column, while the oxygen, still liquid, 
continues its course downward to the receiver. The nitrogen 
gas is delivered into pipe K, travels along pipe L, and fiows 
through the outer jacket of reverse cooler E to the atmosphere, 
or to a holder if it be desired to retain it. 

The oxygen is drawn off by way of an inverted bell P fixed just 
above the surface of the liquid in receiver G, in which position it 
catches a portion of the oxygen vapors just after they are 
evaporated. The oxygen then flows by way of pipe Q, and the 
outer jacket of the reverse flow cooler iV, through the pipe R, 
to a suitable gas holder. Within the coolers E and A^ respectively 
the nitrogen in pipe L and the oxygen in pipe Q are used to cool 
the incoming compressed air, so that by the time they flow 
away from the coolers at M and R they are practically at atmos- 
pheric temperature. 

The above may be considered only a crude outline of the 
original Linde process as actually operated. There are many 
minor details not hinted at, and some features which it is still 
desirable to keep secret. A very important operation is the 
filtering and drying of the air at the beginning. Even with the 
most elaborate precautions all the moisture is not removed, and 
after a plant has been run about six da* it ''freezes up;" but as 
the plants are in dupUcate, one can bex\anning while the other is 
righting itself. 

The Claude Process. — Now to understand the Claude process 
for producing oxygen from liquid air it is necessary to refer to 
Fig. 94 agam to call attention to a fundamental factor of the 
process not so far explicitly mentioned. 



312 



COMPRESSED AIR PRACTICE 



When the apparatus is working steadily a constant stream of 
liquid air emerges from pipe J on to the top tray in the rectify- 
ing column. Simultaneously practically pure liquid oxgyen is 
present in the receiver G. The gas evaporating from this liquid 
and ascending the column is therefore also practically pure 
oxygen. At the foot of the column, then, the temperature is 
that corresponding to the boiling-point of liquid oxygen at 
atmospheric pressure, say, —296.5, and at the top the tempera- 
ture is that of the liquid air, which may be taken as identical 
with the boihng-point of hquid nitrogen under atmospheric 



-320 
100 



80 



40 



20 



■Temperature (Fahr.) 
-315' -310' -305' 



-309' 











/ 




Nitrogen 






/ 








/ 








^ 


Oxygen 




A 




^1 







20 40 60 80 1 

Percentage of Oxygen in Liquid 

Fig. 95. — Diagram of Separation Process. 



pressure, say, — 320, and intermediate between the top and the 
bottom of the column the temperature lies between these limits. 
There is thus a temperature gradient set up in the rectifier, the 
lowest temperature being at the top and the highest temperature 
at the bottom. The iot,ablishment and maintenance of this 
temperature gradient ars v^ssential features of the Linde rectifica- 
tion process, and must be considered a little more closely before 
the Claude improvements can be clearly understood. 

In Fig. 95 a diagram is given graphically illustrating the fun- 
damental points. Suppose that we have a closed vessel partially 
filled with a mixture of liquid nitrogen and Hquid oxygen, and 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 313 

that the temperature of the vessel is somewhere between these 
boiling-points. The space above the level of the liquid will 
contain a vapor composed of a mixture of gaseous nitrogen and 
gaseous oxygen. We might not at first expect this. The tem- 
perature of the vessel being above the boiling-point of nitrogen, 
why should some of the nitrogen still be liquid? So also as the 
temperature of the vessel is below the boiling-point of oxygen, 
why should some of the oxygen be gaseous? 

It may be that these apparently simple questions have never 
been satisfactorily answered, still it is clear that the quantity 
of liquid nitrogen which at the given temperature would be gas- 
eous is in some way balancing a quantity of gaseous oxygen which 
otherwise would be liquid. What experiment tells us about the 
composition of the liquid and its vapor is embodied in the diagram, 
Fig. 95. 

If we start with a liquid containing 100 per cent, of nitrogen, 
the vapor produced will be 100 per cent, nitrogen. So if the 
liquid contains 100 per cent, oxygen the vapor produced will 
also contain 100 per cent, oxygen. It is quite different, however, 
if our liquid is initially composed of n per cent, of oxygen and 
100— n per cent, of nitrogen. The vapor driven off at first will 
contain m per cent, of oxygen and 100 — m per cent, of nitrogen, 
and m in general will not be equal to n, but will be less than n. 
If n = 21, the same percentage composition as the atmosphere, 
the vapor driven off at first will contain only 7 per cent, of oxy- 
gen, as shown by the point A on the diagram. Fig. 95. 

Again, for m to be 21, or to obtain a vapor with the same com- 
position as the atmosphere, we would require to start with a liquid 
containing 48 per cent, of oxygen, as designated by B on the 
diagram. The compositions of these liquids are somewhere be- 
tween pure nitrogen on the one hand and pure oxygen on the 
other, and their boiling-points are between — 320° and — 296°. 
The diagram shows the boiling-point for the liquid containing 
21 per cent, of oxygen to be about — 315°, and for that containing 
48 per cent, about — 308°. 

If a liquid containing n per cent, of oxygen produced a vapor 
containing m per cent, of oxygen, the percentage composition of 
the liquid toward the end of the evaporation would be the same 
as at the beginning, provided m were equal to n. If, however, m 
is less than n the liquid will become increasingly rich in oxygen 
as the evaporation proceeds. If we start with a liquid contain- 



314 COMPRESSED AIR PRACTICE 

ing 21 cu. ft. of oxygen and 79 cu. ft. of nitrogen, then according 
to the diagram, by the time we have evaporated 1 cu. ft. of the 
liquid we have removed from the liquid 0.07 cu. ft. of oxygen 
and 0.93 cu. ft. of nitrogen. Out of the 99 cu. ft. of liquid 
remaining 20.93 are oxygen and 78.07 are nitrogen. The per- 
centage of oxygen in the liquid has risen from 21 to 21.14, and 
that of nitrogen has fallen from 79 to 78.86, and since the oxy- 
gen percentage has thus increased, the boiling-point will have 
risen to correspond. Hence, unless the temperature at which 
the evaporation is commenced is increased as time goes on, 
the liquid will cease to evaporate. 

In Fig. 94, at the top of the column issuing from pipe J we have 
liquid air at a temperature of about — 320°. As it falls down the 
column it experiences the influence of the temperature gradient 
and its temperature rises until it reaches the boiling-point of 
liquid air, —315°. Evaporation then commences and vapors are 
given off containing 7 per cent, of oxygen. The composition of 
the liquid begins to change; its percentage of oxygen increases 
and its boiling-point rises. The temperature gradient, however, 
also is rising, so that the descending stream of liquid is being 
continuously evaporated from the top to the bottom of the 
column. 

Side by side with this process another is going on, namely, 
the condensation of the gaseous oxygen ascending from the 
receiver. How do these two processes affect one another, and 
what is the net composition of the vapor at any given point in 
the column? The mechanism of the interchange which goes on 
is difficult to state in concise and exact terms. The ultimate 
result is that at any given point in the rectification column the 
composition of the resultant vapor and the composition of the 
resultant liquid are functions of the temperature of the point, the 
relationship being as exhibited in Fig. 95. In other words, if a 
liquid containing n per cent, of oxygen and 100 — ri per cent, of 
nitrogen be intimately mixed with a vapor containing m per 
cent, of oxygen and 100 — m per. cent of nitrogen, and with another 
vapor consisting of 100 per cent, oxygen, simultaneous condensa- 
tion and evaporation will proceed until a resultant liquid and a 
resultant vapor are formed the compositions of which are as 
indicated in Fig. 95 under the temperature at which the mixing 
process is conducted. 

When the steady state has been reached the vapor in the 



I 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 315 

rectification column has at any given point a constant composi- 
tion. It is pure oxygen at the foot; at the top is a mixture of 
7 per cent, oxygen and 93 per cent, nitrogen and up and down 
the column is a regular graduation from one of these to the other. 
The vapor is therefore drawn off at the top of the column, for it 
is here that the oxygen percentage is least. The vapor drawn 
off into the pipe K, Fig. 94, is not pure nitrogen. It contains 7 
per cent, of oxygen, or that percentage of oxygen which is found 
in the vapor arising from normal liquid air. 

In the Linde process this loss of oxygen cannot be avoided. 
To obtain a less percentage of oxygen in the nitrogenous vapors 
drawn off through pipe K we would require something else than 
liquid air issuing from pipe /. We would in fact need to have 
a liquid richer in nitrogen and poorer in oxygen than is liquid 
air. 

Broadly speaking, the Claude process consists in dividing 
the liquid air, before it reaches the rectification column, into 
two portions, one poorer in oxygen and the other richer in oxygen 
than the original liquid air. Each portion is introduced into 
the rectification column at the point on the temperature gradient 
corresponding to its respective boiling-point. The nitrogenous 
vapor carried off in the process still contains oxygen, but the 
percentage is less then in the Linde process, and, in fact, corre- 
sponds with the percentage contained in the evaporate of the 
liquid fraction poorest in oxygen. 

Fig. 96 is a diagram of the Claude process drawn so as to be 
readily comparable with Fig. 94, the diagram of the Linde process. 
It will be noticed that, starting from the left, Fig. 96 is similar 
to Fig. 94 until the point S is reached. At this point a branch 
pipe passes upward and leads into a vessel called a liquefier. 
The main pipe at S, however, turns down and leads into an 
expansion engine, the exhaust from which is conducted to the 
rectification column. The by-pass going through the liquefier 
joins up to the exhaust pipe of the expansion engine through 
a throttle valve I. We first suppose that this valve is shut, so 
that the liquefier is out of action, also that valves A, B, and C 
are shut. 

Under these circumstances, air from the compressor enters 
the system at D and passing down the reverse flow cooler E, 
reaches the point S. As the by-pass is, for the time being, a 
blind pipe, all the compressed air has to find its way through 



^ 



316 



COMPRESSED AIR PRACTICE 



the expansion engine, which is coupled to a djniamo and made 
to do external work and the temperature of the exhaust is there- 
fore low. The cold expanded air flows into a vessel U, from 
which two tubes pass upward into a vessel V, which in turn is 
conducted by two tubes passing downward with an annular 
vessel W. From W the cold air jflows past the valve X, wide 
open at this time, up the pipe J, and into the interior of the 
rectifier. Part of the cold air leaves the vessel U by an alter- 




Expansion- — [ 
Engine 



Fig. 96. — The Claude Oxygen Process. 

native route consisting of a pipe Z leading from the foot of the 
vessel U through a wide open valve Y into the rectifier. 

Both portions of cold air leave the rectifier by way of pipe X, 
and, flowing through the outer chamber of the liquefier, reach 
the reverse flow cooler E, whence they are conducted by the pipe 
M back to the compressor. The compressed air delivered to 
the expansion engine thus, as time goes on, becomes progressively 
cooler, and the expansion engine is fullfilling precisely the same 
duty as the throttle valve H, Fig. 94, of the Linde apparatus. 
It is also clear that during this preliminary stage of the operations 
the rectifying column, the vessels U, V, W, and the liquefier 
are all being cooled down by the cold exhaust of the expansion 
engine. 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 317 

When the liquefier has been cooled sufficiently, valve T is 
opened and compressed air is by-passed from the point S. 
This air on issuing from the throttle valve T is converted into 
liquid, and as such is carried along the exhaust pipe by the ex- 
haust from the engine into vessel U. It is not, however, allowed 
to remain here, but is swept up pipe Z by the exhaust into the 
interior of the rectifier. Falling downward it starts to collect 
in receiver G round the pipes leading into and out of vessel V. 
When sufficient liquid air has collected in the receiver, valves 
A, B, and C are opened and valves X and Y are partially closed 
so as to produce a throttling action. 

Under these changed conditions the air reaching point S as 
before now divides, part going through the liquefier and part 
through the expansion engine. The two portions unite in the 
exhaust pipe and reach vessel U partly as liquid and partly as 
gas. This saturated vapor passes up the innermost pair of tubes 
into vessel V. In so doing it is cooled by the surrounding 
liquid air and suffers partial condensation. Since oxygen is 
more readily condensible than nitrogen it is clear that the 
liquid condensed at this stage will be richer in oxygen than is 
liquid air. Such rich liquid falls back into vessel U. The 
remaining gaseous portion of the air is naturally poorer in oxygen 
and richer in nitrogen by the result of this partial condensation. 
From vessel V it flows down the outer pair of tubes into the 
annular chamber W. Further condensation takes place, and 
as a result a liquid poor in oxygen collects in the chamber W. 

Under the pressure of the exhaust of the expansion engine 
the liquids rich in oxygen and poor in oxygen are forced up pipes 
Z and /, respectively, past throttle valves Y and X into the 
rectifier. It will be noticed that pipe J enters the column 
at the top and pipe Z about halfway down. Inside the column 
the two portions of liquid come in contact with the vapors 
arising from the liquid air in receiver G. An exchange takes 
place, the ascending vapers growing richer in nitrogen and the 
descending liquids richer in oxj'gen. As time goes on the liquid 
in receiver G loses more and more nitrogen until, when the 
steady state is reached, it is pure, or practically pure, oxygen, 
just as in the case of the Linde apparatus previously described. 

With the attainment of this stage a temperature gradient 
has been definitely established within the rectifier. At the foot 
there is the temperature corresponding to the boiling-point of 



318 



COMPRESSED AIR PRACTICE 



liquid oxygen under atmospheric pressure. At the top there 
is the temperature corresponding to the boiling-point of the 
liquid ascending pipe /. As this liquid is poorer in oxygen than 
is liquid air, the temperature at the top of the Claude rectifier 
will be less than at the top of the Linde. Between the top 
and the bottom of the Claude rectifier there is one point on the 
temperature gradient which corresponds with the boiling-point 
of the liquid ascending pipe Z, the liquid, that is, which is rich 
in oxygen. Care is taken that pipe Z is located to enter the 
column just at this point. 

In practice it is found that out of every three volumes of air, 
measured in the liquid state, entering vessel U, one volume 
passes up pipe Z as rich liquid, and two up pipe J as poor liquid. 
The composition of the rich liquid is roughly half oxygen and 
half nitrogen, while that of the pure liquid is about 6 per cent, 
oxygen and 94 per cent, nitrogen. The rectification process 
is precisely the same fundamentally in the Claude plant as in 
the Linde. At any point on the temperature gradient the 
composition of the vapor is in accordance with the data of Fig. 95. 
It follows, then, that at the top of the column, where the liquid 
contains only 6 per cent, of oxygen, the vapors driven off contain 
about 2 per cent, oxygen arid 98 per cent, nitrogen. These 
nitrogenous vapors are carried off along pipe K and pass in 
succession through the outer jackets of the liquefier and the 
reverse flow cooler £', being finally allowed to escape. They 
contain 2 per cent, of oxygen, as compared with i\\Q 7 per cent, 
of the Linde process. The oxygen gas is drawn off from receiver 
G by way of pipe Q, and after being passed through the outer 
jacket of the reverse flow cooler N is conducted, by pipe R, to a 
suitable holder. 

The purity of the gas produced by the Linde and the Claude 
processes is guaranteed as between 98.5 and 99.5 per cent. 
It is well known, however, that the purity is frequently as high 
as 99.8 per cent. It is claimed that the liquid-air process is 
the only means whereby oxygen entirely free from combustible 
residuals can be produced. 

It is to be expected, perhaps, that some readers will not 
carefully go entirely through the preceding somewhat intricate 
description. Those who do cannot fail to reaUze at once its 
incompleteness. Many practical details of construction and ar- 
rangement, and of the means of manipulation and control are not 



LIQUID AIR— OXYGEN FROM THE ATMOSPHERE 319 

even suggested. The sketches are necessarily misleading as to 
the relative and actual dimensions and capacities of the different 
elements, as for instance those of the compressor in Fig. 93 and of 
the expansion engine in Fig. 96. No information is given as to 
actual pressures and temperatures at the different points. One 
may well wonder and inquire how the man in charge of the 
apparatus can keep himself informed as to all that is going on 
throughout the series, even when everything is going well, and 
how he can discover when things are going wrong or determine 
what he should do to right them. 

It is to be noted that nitrogen, the largest constituent of 
the atmosphere both in weight and bulk, is here represented as 
a discarded by-product, which it was completely in the beginning, 
but, like many other by-products, its value and the means and 
methods for its appropriation are being developed, which may 
ultimately mean not only nitrogen cheap and plenty, and a good 
demand for it, but in consequence still cheaper oxygen. With 
the clamor for nitrates for fertilizers the possibilities here opening 
cannot be ignored. By means of a simple evaporative device, 
several of which are already in use, it is possible without addi- 
tional expenditure of power to eliminate the last traces of oxygen 
from part of the nitrogenous vapors coming away from the 
Linde and Claude rectifiers. In some cases, it is stated, plants 
have been established already for the production of nitrogen in 
the first instance with oxygen as the by-product. 




INDEX 



Absolute temperature, 14 
Account of excessive use of oil, 199 
Adiabatic, 18 

and isothermal curves, to draw, 
55 
Afreet, the, at Riverside Drive, 202 
Aftercoolers, 161 
Air a jack-of-all- trades, 2 

always warmed in entering the 

cylinder, 69 
bladder of the fish, 3 
blast in foundation sinking, 293 
card reverse of steam card, 50 
compression more complex than 

water pumping, 94 
consumption of rock drills, etc., 

143 
for keeping ships afloat, 286 
for large steam hammers, 266 
for raising ships, 285 
instead of steam for mine hoists, 

164 
jets, 298 

lift, advice and experience 
needed for installing, 260 
can ignore power economy, 

252 
demands adequate submer- 
gence, 253 
formula for air consumption, 

258 
for special fountain effects, 

262 
for steady work, 259 
free air requirements for 
different submergences, 257 
incorrect theory of layers or 

pistons of air, 255 
no mystery about operation, 

252 
operates by gravity, 254 
submergence percentages, 256 
typical discharge from, 263 
uses air isothermally, 255 

23 



Air lift, volume of free air decreases 
as submergence increases, 
259 
wide use of, 252 
must get inlo the cylinder cool, 

69 
need not pass through the re- 
ceiver, 167 
not used expansively in steam 

pumps, 244 
protection schemes of W. W. 

Wothcrspoon, 291 
receiver fires and explosions, 189 
should be strong enough 

when red hot, 194 
sometimes a combustion 
chamber, 189 
similar to steam in action, 11 
vapor of a liquid, 11 
All air is compressed air, 1 
Ancient methods of gas distribution, 

201 
Animal life promotes plant growth, 

8 
Anomalous vacuum readings, 22 
Atmosphere only lends its constit- 
uent gases, 7 
Automatic restorative processes, 7 
speed regulation, 100 

Baffle plates in receivers, 163 

Barometer, 19 

Beneficent gas holder cheapens 

living places for the poor, 

204 
Blow-off of Taylor compressor, 126 
Boy can blow the gas out, 206 
Building specifications of fighting 

craft, 292 
Bulge of head reversed by explosion, 

191 
Bulkheads of the Titanic, 2S7 

should be covered by deck 

above, 280 



321 



322 



INDEX 



By what right? 

Caisson a stationary diving bell, 279 

diving bell and, 278 
Caissons for foundations of build- 
ings, 281 
for subaqueous tunneling, 282 
for tunnel under the Seine, 278 
Catskill aqueduct has head and 
volume for fountain effects, 
262 
monument, for a, 260 
Cards showing successive steps of 

unloading, 105 
Cement gun, 299 

description of operation, 300 
invention of a taxidermist, 301 
Centrifugal blower possibilities, 114 
Chinese air receivers, 194 
Choking the intake, 101 
Circulation of cooling water, 97 
City water main for constant air 

pressure, 168 
Claude process for abstracting oxy- 
gen, 311 
Clearance controllers, 103 

losses in air cylinder of pump, 
243 
Competitors' secrets, 121 
Compressed air by the pound, 23 
for raising water, 240 
motor, 66 
problem, the, 26 
protection of U. S. ships, 290 
storage, 164 
Compressing natural gas, 212 
Compression computations, 33 
Compressor claims gas employment, 
213 
its own dynamometer, 49 
leakage as a detail of ineffi- 
ciency, 143 
Compressors for gas distribution, 
206 
in Pittsburgh district, 215 
Condition of air from turbo com- 
pressor, 120 
of pipe surfaces, 179 
Constant lift return air system, 249 



Constant pressure by water reser- 
voir, 165 
with varying consumption, 
100 

Contrasted effects of black powder 
and dynamite, 229 

Controlled inlet valve closing, 103 

Cool air necessary for cheap com- 
pression, 67 

Cooling during compression, 75 

systems in gasoline practice, 219 

Cost of air for steam hammers, 268 
of Taylor compressor, 126 

Cylinder surfaces differently heated, 
73 

Danger from longitudinal bulkheads, 

287 
Day's record of high pressure gas 

transmission, 211 
Deposit of oil residuum in receiver, 

189 
Detailed losses in compression, 139 
Details of successive strokes of 

Humphrey compressor, 136 
Diagram of gas separation process, 

312 
of Claude oxygen process, 316 
Different m.e.p. for compression and 

for delivery, 47 
mountings for the rock drill, 225 
thermal cost of reheating and 

of compressing, 182 
Direct displacement pumps, 245 
Distribution of the two-stage load, 

79 
Draining by pockets in depressions 

of the line, 162 
Drill sharpener, 230 

changes attitude of driller, 230 
Drive of the compressor, 108 
Driving steam pumps with air, 242 
Duty of successive impellers, 117 

Earth's air and water power, 2 
Economy in compression begins at 

the beginning, 66 
Efficiency of intercooling, 97 

test of three-stage machine, 98 



INDEX 



323 



Eight inch globe, 5 

Electric air drill, 232 

air chases the piston, 234 
at Kensico dam, 235 
comparison with regular drills, 

238 
critical point in feeding, 235 
description of drill, 233 
drill wagon, 238 
for channeling, 235 
name misleading, 232 
no freezing up, 235 
oihng the drill, 235 
power cost of drilling, 238 
pulsator runs at constant speed, 

235 
saves much power, 235 
simplifies drill construction, 234 
solves a difficult problem, 232 
strikes a hard blow, 234 
truck, 233 

will not let bit stick, 235 
works at all altitudes, 235 

Electricity jumps in, 109 

English examples of high pressure 
gas, 208 

Fainting mermaid, 4 

False air receiver assumptions, 160 

Final inrush of air as compression 
begins, 63 

Fires smothered by closed bulkhead 
system, 292 

Flames carried long distances in 
pipes, 189 

Fountain effect of air lift, 264 

Fountains most effective monu- 
ments, 261 

Free air the basis of compression 
computations, 66 

Future of turbo compressor, 119 

Gas and air transmission formulas 
only approximate, 173 
holder abolition and real estate 
booming, 205 
business would not now start 
with ounces, 205 



Gas holder could not carry 1 lb. 
pressure, 206 
^ depreciates property, 203 
does not ask the right, but 

takes it, 201 
was started wrong, 209 
Gasol, 221 
Gasoline business hazardous, 221 

by compression, 216 
Given volume of air does less work 

than steam, 149 
Great heating and lighting power of 
gasol, 222 

Hammer, compound air, 272 
Massey, 274 
Musker, 274, 276 
Hammers, types in use, 272 

steam drive bad in itself, 266 
various air operated, 269 
work better with air, 266 
Heads torn off rivets, 191 
Heat of air in clearance space, 72 
of compression abstracted at 
end of stroke, 76 
Heated air and leaky discharge 

valves, 74 
High explosives in mining, 228 

pressure gas and railway trains, 
208 
an object lesson in, 209 
at San Francisco, 207 
for Cincinnati, 213 
for cities, 200 
Higher pressure and quicker flow, 

207 
High temperature in portable 

receiver, 196 
Hippodrome march into the water, 

278 
Hoisting water with a bucket, 250 
Horse-power from m.e.p., 53 
Humphrey pump, 128 

as an air compressor, 135 
large installation of, 137 

Ideal conditions for turbo com- 
pressor drive, 120 



324 



INDEX 



Improving cities destructive as well 

as constructive, 202 
Increase of pressure by reheating, 

186 
Indicator card, interpretation lines 
on, 52 
tells whole story of compression, 

49 
theoretical, 51 
two-stage compression, 61 
undulations of dehvery Hne, 62 
Inlet valve opening, 103 

step by step regulation, 102 
Intake air, filtering and cooling, 67 

with Httle heating of air, 71 
Intercooling to within two degrees, 

163 
Inventions successively superseded, 

119 
Isothermal, 18 

compression the ideal, 29 
in Taylor compressor, 126 

Jack hammer drill, 227 
Johnson-Rix formula, 174 

Labor saved as well as fuel, 110 
Large contractors learning the big 
economies, 108 
storage of natural gas impossible, 
214 
Layout of gasohne plant, 217 
Leaky discharge valves, 73 
Limit figure for flow in pipes, 178 
Limited ratio for explosive mixtures, 
189 
range of reheating, 183 
Linde liquid air plant, 307 

oxygen plant, 309 
Liquefied natural gas, 216 
Liquid air, 305 

at first a disappointment, 305 
critical temperature, 305 
value for separating oxygen, 306 
Loss diminishes as pressure increases, 

119 
Losses not chargeable to air, 244 

small in pipe transmission, 173 
Low specific heat of air, 181 



Lubricants used in Panama com- 
pressors, 198 
Lubrication better with oil, 267 

Mechanical wonder, a, 120 
Mercury pressure gage, 21 
Michigan copper mining, 228 
Multi-stage cy Under ratios, 80 

Natural functions of the air, 1 
Nearly perfect indicator card, 78 
N. Y. water tunnel compressors, 112 
No satisfactory reheaters, 187 

special engine required for air 

drive, 149 
waste of water used for air 
pressure, 167 

Oil wells are gas wells, and vice 
versa, 216 

One-man drills, 227 

Operation of direct air pumping, 
248 

Oxygen absorbed more readily than 
nitrogen, 128 
better understood than nitro- 
gen, 13 

Peak gas load, 207 
Perfect clearance control, 107 
Picturesque for the engineer, 95 
Pint of oil for 96 acres of surface, 198 

in 24 hours, 198 
Pipe sizes a compromise, 169 

transmission, 169 
Pneumatic painting, 299 

tools on caisson work, 284 
return to primitive practice, 
226 
Portable liquid gas, 221 
Possible efficiencies are impossible 
efficiencies, 243 
percentages of gain by reheat- 
ing, 185 
Power cost of compressed air, 138 
from compressed air, 149 
required for raising water, 240 
Precision in rock drill manufacture, 

224 
Pressure in gas wells soon drops, 217 



4 



INDEX 



325 



Process of gasoline production, 217 

Quantity of lubricant for com- 
pressors, 198 

Rapid cooling of air in main, 112 
making of compressor history, 

119 
wear of sand blast nozzles, 296 
Ratios of m.e.p. to volume ratio, 38 
of pressure, volume and tem- 
perature, 33 
Receivers do not cool the air, 160 

dry the air, 161 
Reduction of capacity by rise of 

intake temperature, 70 
Regulating devices for compressors, 

99 
Reheating compressed air, 181 

is doing work over again, 182 
Relations of pressure, volume and 

temperature, 15 
Relative capacities of distributing 

pipes, 171 
Return air pumping, 245 

plant, 250 
Rise of temperature coincident with 

compression, 18 
Rivets should be banished, 195 
Rock drill developments, 223 

leads the strenuous life, 225 
Rotation of rock drill, 224 
Rotors of fan drills and of turbo 

compressors, 115 
Run-around regulation, 101 

Safety valve and throttle control, 100 
Sand blast, cleaning buildings, 290 
extensive use of, 294 
for the bath, 298 
in art work, 296 
low or high pressure?, 295 
protecting the workman, 295 
sand must be dry, clean and 
sharp, 295 
Saving by compressed air at Butte, 

164 
Separators required for super-satu- 
rated air, 162 



Single-stage compression, 43 
Skip valve, 102 

Small range of atmospheric pres- 
sures, 6 
Specific heat, 17 
Spontaneous ignition in receivers, 

195 
Stage compression saving wasted at 

the steam end, 108 
Static water potentials, 241 
Steam charges for waiting, air only 
for working, 267 
consumption of compressors, 144 
drive abandoned for electricity, 

109 
guarantees its own temperature, 

70 
separators effective for air, 162 
used for air reheating without 
blowing off, 187 
Still-hunt of turbo compressor 

builders, 121 
Storage value of receiver small, 160 
Strength of pipe, 180 
Study of three-stage compressor, 94 
two-stage tandem compressor, 
82 
Submarine diver, 285 

Taylor compressor, 122 

takes oxygen out of the air, 127 
the reverse of the air lift, 122 
Temperature of air with clearance 
controller, 104 
rise in compression not uniform, 
18 
Tempering in oil, 196 
Test of pipes by Crane & Co., 180 
Thermometer record of typical igni- 
tion, 197 
Thinking to scale, 5 
Three types of machine drills, 226 
Turbo-compressors, 114 

on the Rand, 120 
Two-stage compression, 79 

cross compound equalizes the 

load, 94 
tandem shows how not to do it, 
88 



326 



INDEX 



Typical electric driven compressor 
plant, 111 
receiver explosion, 190 
stunt with jackhammer, 228 

Vacuum in gas lines important in 
gasoline production, 220 

Valve arrangement for return air 

pumping, 251 

motion of the rock drill, 224 

Variable hft return air pumping, 249 

Velocity hmit in pipe transmission, 
171 

Vertical receiver raised off its feet, 
194 

Volume a factor in pipe transmis- 
sion, 169 



Volume, and velocity of flow, 171 
cheaply obtained by reheating, 

181 
of air in compression, 53 
delivered, 54 

Water injection for cooling air, 77 
wets and clings to surfaces, 162 
under steam pressure for reheat- 
ing, 187 
Waterwheels are windmills, 2 
Welding of pipe line joints, 195 
What is required of the rock drill, 

223 
What the air carries, 3 



mtm 



NOV 21 1913 



